WO1983003007A1

WO1983003007A1 - Method and device for determining hydrogen flux

Info

Publication number: WO1983003007A1
Application number: PCT/DK1983/000022
Authority: WO
Inventors: A/S Hellesens
Original assignee: Arup, Hans
Priority date: 1982-02-26
Filing date: 1983-02-25
Publication date: 1983-09-01
Also published as: JPS59500331A; NO833886L; EP0103588A1; DK492183D0; JPS59500063A; DK492183A; JPH0318112B2; AU1331083A

Abstract

A device (10) for determining hydrogen flux through a metal membrane comprises two parts, top part (12) and a bottom part (11) respectively. The top part (12) is electrically insulated relative to the bottom part (11). The top part (2) which is a sheet of iron or steel, is provided with an interior palladium coating (13). The top part (12) constitutes together with the interior coating (13) a metal membrane which is selectively permeable to hydrogen. Within the bottom part (11) which is made of a metal, e.g. nickel, or nickel-coated steel, a metal oxide filling (14) is arranged. The bottom part (11) constitutes together with the metal oxide filling (14) a metal/metal oxide cathode. The device is also provided with an electrolyte solution (17) electrolytically communicating with the metal membrane (12, 13) and the metal/metal oxide cathode (11, 14). The device is adapted to be maintained in a substantially short-circuited condition by either a direct connection between the metal membrane and the metal/metal oxide cathode or a connection to a current measuring system with low internal impedance, such as a conventional am-meter.

Description

METHOD AND DEVI CE FOR DETERMIN I NG HYDROGEN FLUX

The present invention relates to a method and a device for deter¬ mining hydrogen flux through a metal membrane.

TECHN ICAL BACKG ROUND

I n some chemical process industries, corrosion of steel components may occur under conditions which give rise to hydrogen-induced cracking or blistering . A typical and technically important example is corrosion of pipes and pressu re vessels for natural gas containing a condensed liquid phase containing water and hydrogen sulphide (along with other substances) . This condensate is slightly acidic, and will corrode steel by the generation of hydrogen . The presence of hydro¬ gen sulphide inhibits the formation of molecular hydrogen, and a significant part of the hydrogen permeates into the steel where it may cause damage in the form of cracking or blistering .

It is possible to reduce the hydrogen damage in several ways , e. g . by avoiding condensation , by removal of hydrogen sulphide or by ad¬ ding inhibitors which may perform their function in several ways, e. g . by neutralizing the liquid phase or by forming a protective film.

I n order to be able to monitor the effect of these precautions, the hydrogen uptake or rate of hydrogen uptake in an exposed area of the steel component in question should be measu red . This can be done in accordance with several known measuring principles, most of which are based on the fact that the hydrogen which permeates into the exposed entry side of a steel wall, will diffuse th rough the steel wall and be liberated in gaseous (molecular) form on the side of the steel wall opposite to the entry side. The rate of hydrogen generation on said opposite side of the steel wall may be measu red by means of known techniques, e. g . by measuring the development of pressure in a closed cavity or by measu ring the displacement of a liquid drop in a glass tube having a calibrated inner wall : The amount of hydrogen may also be analysed in various ways, e. g . by gas phase chromato- graphy.

To carry out the measurement of the rate of hydrogen generation in accordance with the above-mentioned physical measuring principles several hydrogen measuring probes have been proposed. However, these hydrogen measuring probes suffer from various drawbacks . Some of them are rather primitive and have low sensitivity, while others need constant supervision and are not suited for automated datalogging . Others are more sensitive and accurate, but are quite costly and delicate.

An electrochemical measuring principle based on electrochemical oxi¬ dation of hydrogen which permeates to the side of the steel wall op¬ posite to the entry side is also known . This technique was first de- scribed by N . A.V. Devanathan and Z. Stachorski (Reference 1 ) . A hydrogen probe or hydrogen cell constructed in accordance with these principles is known as a Devanathan-cell .

In this cell an iron or steel sheet forms a wall or membrane between two compartments . A first compartment constitutes an experimental compartment in which the rron or steel material of the wall is exposed to corrosive forces or other experimental influences which are likely to produce hydrogen in the iron or steel material . A second compart¬ ment contains a de-aerated al kaline solution, e. g . 1 molar potassium hydroxide. The surface of the iron or steel wall facing the second compartment is covered with a thin coating of electrodeposited palla¬ dium which is permeable to hydrogen and stable in the alkaline solu¬ tion . A platinum counterelectrode is arranged opposite to the iron or steel wall , and a reference electrode is also arranged in the second compartment having its tip arranged in close proximity to the palla- dium-covered membrane. I n order to maintain a constant potential of the palladium-coated iron or steel wall in the alkaline solution , a potentiostat is used . Generally, a potential of E, . = ⁺100 mV is used . At this potential , the palladium coating and the iron or steel material of the wall are passive. Therefore, the passive current density is quite "small, typically between 0.01 and 0.1 yA per cm². Any hydro¬ gen diffusing through the iron or steel wall and the palladium coating thereof is oxidized at the side of the palladium coating opposite to the entry side of the iron or steel wall in accordance with the following equation:

H ⁺ OH^{" •}* H₂O ⁺ e^".

Consequently, an increase of anodic current in excess of the passive background current is equivalent to a hydrogen flux through the wall. The hydrogen flux through a unit area of the iron or steel wall of a given thickness is a product of the hydrogen activity at the external side of the wall and the diffusion admittance through the wall, assuming that the potential is capable of maintaining a hydrogen activity at approximately zero at the opposite, i.e. the internal palla¬ dium-coated side of the wall, if the diffusion admittance is known, the hydrogen activity in the first compartment may be calculated on the basis of the anodic current of the cell. The diffusion admittance is determined by having a controlled electrochemical environment in the first compartment or by introducing a hydrogen atmosphere of known pressure into the first compartment, the external side of the iron or steel wall being coated with palladium which renders the cell sensitive to molecular hydrogen.

Another laboratory measuring device based on the Devanathan measur¬ ing principle is described in Reference 2. In this measuring device, a steel plate constitutes a part of a wall of a cell containing an electro- lyte solution, a reference electrode and a counterelectrode. Further¬ more, a potentiostat is needed to operate the device.

GB Patent No. 1.585.070 discloses an electrochemical cell for determi¬ ning the concentration of hydrogen in a fluid, which electrochemical cell is based on the principles of the Devanathan measuring tech- nique. The cell comprises a container for an electrolyte solution, a working electrode constituting a wall part of the container and being adapted to be exposed to a fluid under test, and a platinum/platinum oxide reference electrode having an area comparable to the working electrode and being arranged close to and parallel to the working elec- trode.\ In one embodiment of the cell, a counterelectrode, means for maintaining the working electrode at a fixed potential relative to the reference electrode, and means for determining the current flowing through the working electrode are provided. This embodiment has to be used in combination with electronic means for maintaining the working electrode at a fixed potential . I n another embodiment of the measuring cell, the electronic means, the counterelectrode and the current determining means are omitted . I nstead, the working electrode and the reference electrode are connected to potential measuring means. According to the disclosure of GB Patent No. 1 385 070, a potential difference between the electrodes is related to the concen¬ tration of hydrogen at the working electrode. However, the specifi¬ cation explains that a stable reading is obtainable only at low hydro¬ gen activities .

From GB Patent No. 1 .524.017, another hydrogen measuring cell based on the same measuring principles is known . The cell (a so- called "Patch Cell") is adapted to be secu red in fluid-tight engage¬ ment with an outer surface of a steel or iron wail, the interior side of which is exposed to corrosion . The cell responds quantitatively to all hydrogen escaping from the outer surface of the steel or iron wall, thus measuring the amount of hydrogen produced on the inner sur¬ face of the steel or iron wall .

In an article by F. Mansfeld et al . (reference 5) , a measuring system based on the above described Devanathan measuring principle is disclosed . The system comprises a measuring cell consisting of a cell body which is press-fitted into a cylindrical magnet assembly serving fixation purposes when brought into contact with a metal sample body. The cell body contains a cavity which opens into the side of the cell body, which is adapted to be brought into contact with the surface of the metal sample body. I n the cavity of the cell body, a Ni-NiO counterelectrode is arranged together with an additional Ni-NiO electrode which serves to check the potential of the counter¬ electrode and the presence of a sufficient amount of electrolyte. The electrolyte is also arranged in the cavity and is absorbed in a cellu- lose sponge which is positioned in the cavity of the cell body. The cellulose sponge constitutes a carrier body for the electrolyte which on one hand is in direct contact with the Ni-NiO counterelectrode and on the other hand is brought into direct contact with the surface of the metal sample body when the cell is arranged in contact therewith . I n this measuring cell, the reference electrode, the potentiostat and the counter electrode of the above described Devanathan-cell are combined into a single Ni-NiO counterelectrode. The measuring system further comprises an electronic measuring and timing circuit including a current follower, an instrument panel and four timers . The entire system is powered by batteries having a six-hour operation capacity. The measuring system is intended to be used for determination of hydrogen concentration in steels, based on the electro-chemical per¬ meation technique. As stated in the article, the cu rrent generated in the measuring cell contains transient contributions having their origin in a passivation process immediately after the cell has been

-1/2 assembled . While the permeation current itself is a function of t , these transient contributions decay with t . The sensitivity of the

2 cell is approximately 1 yA/cm . While the fi rst and the second timers of the electronic measu ring and timing circuit serve the purpose of elimination of the above transient passivation contributions, the third and the fou rth timers control a digital integrator which carries out a digital integration of the cu rrent supplied from the measuring cell within a predetermined time period determined by said third and fourth timers . As stated in the article, the integration of the cu rrent supplied from the measuring cell within a predetermined time period may be employed for determination of the original concentration of hydrogen in the metal sample body. As will be understood from the above, the system is merely adapted to provide a single determination of an original hydrogen concentration of a metal sample body, and is not adapted to provide continuous monitoring on an actual measuring site for a long period of time. Fu rthermore, the system has to be assembled prior to use and, consequently, is not a permanently sealed, ready-to-use system. The system also relies on a highly elaborate electronic measu ring and timi ng ci rcuit. BRIEF DESCRI PTION OF THE INVENTION

The method and device of the present invention permit a stable and reliable measurement of hydrogen flux by means of units, which may be produced at low cost, and which require no maintenance and use simple and reliable external measuring equipment.

The invention provides a device for determining hydrogen flux through a metal membrane, which is selectively permeable to hydro¬ gen, from a hydrogen entry side, comprising the metal membrane, an electrolyte solution communicating electrolytically with the side of the membrane opposite to the hydrogen entry side, a metal/metal oxide cathode communicating electrolytically with the electrolyte, the cathode being capable of oxidizing hydrogen into hydrogen ions, the electro¬ lyte and the cathode being contained in a housing of which the metal membrane constitutes a wall portion, and leads for electrical connec- tion to external current measuring equipment, the device being adapt¬ ed to be maintained in a substantially short-circuited condition by direct connection between the leads or by connection to a current measuring system with low internal impedance. The measuring prin¬ ciple utilized according to the present invention is the above-de- scribed Devanathan principle. However, according to the invention, the functions of the reference electrode, the potentiostat and the counterelectrode have been combined into one single component, i.e. the metal/metal oxide cathode.

Thus, the measuring device of the invention only involves the use of two electrodes, one of which is constituted by the membrane, which renders it possible to make a compact, simple and accu rate measuring unit. For carrying out the measu rement, the only external equipment required is an am-meter. For the sake of proper functioning of the device according to the invention, it is of mandatory importance that the device be always kept in a short-circuited state, whether in use (short-circuiting through the am-meter) or not in use (short-cir¬ cuiting through a direct connection between the leads) .

O PI A particular aspect of the present invention is a low cost and compact device for determining hydrogen flux th rough a metal membrane, which is selectively permeable to hydrogen , from a hydrogen entry side, comprising the metal membrane, an electrolyte solution commu- nicating electrolytically with the side of the membrane opposite to the hydrogen entry side, and a metal/metal oxide cathode communicating electrolytically with the electrolyte, the cathode being capable of oxidizing hydrogen into hydrogen ions, the electrolyte and the catho¬ de being contained in a housing of which the membrane constitutes a wall portion , the housing being permanently sealed . I n this aspect of the present invention, a measuring device having extremely small dimensions, including small internal volume, may be provided . Due to the permanent sealing, a permanent high standard of purity and reproducibility may be obtained by finishing the measu ring device in a factory under controlled conditions, and no maintenance is re¬ quired, as contrasted to known hydrogen measuring devices .

The invention also relates to a method for determining hydrogen fl ux through a metal membrane, which is selectively permeable to hydro¬ gen, from a hydrogen entry side_/ comprising: providing a housing, arranging a metal/metal oxide cathode and an electrolyte solution in the housing, which cathode is capable of oxidizing hydrogen into hydrogen ions and communicates electrolyt¬ ically with the electrolyte solution, arranging in the housing the metal membrane so that it constitutes a wall portion thereof, contacting the side of the metal membrane opposite to the hydrogen entry side with the electrolyte solution , sealing the metal membrane in the housing, short-circuiting the metal membrane at the metal/metal oxide cathode, and measu ring the electrical current generated as the result of oxida¬ tion of hydrogen passing th rough the membrane.

Another aspect of the present invention relates to a method for determining hydrogen flux through a metal membrane, which is selectively permeable to hydrogen , from a hydrogen entry side comprising: providing a housing, arranging a metal/metal oxide cathode and an electrolyte solution in the housing , which cathode is capable of oxidizrpg hydrogen into hydrogen ions and communicates electrolytically with the electrolyte solution, arranging the housing in contact with the metal membrane so that at least part of the metal membrane constitutes a wall portion of the housing, contacting the side of the metal membrane opposite to the hydrogen entry side with the electrolyte solution, sealing said part of the metal membrane relative to the housing, short-circuiting the metal membrane and the metal/metal oxide cathode, and continuously measuring the electrical current generated as the result of short-circuiting the metal membrane and the metal/metal oxide cathode. I n this aspect of the present invention, the escape of hydrogen from the side of the metal membrane opposite to the hydrogen entry side is determined con¬ tinuously by continuously measuring the electrical current. On the basis of escape of hydrogen , the hydrogen flux through the metal membrane may be determined .

When carrying out the method according to the invention , the above device according to the invention may advantageously be employed providing highly reproduceable and accu rate measuring results .

DETAI LED DESCRI PTION OF THE I NVENTION

The metal membrane used according to the invention should be of a metal which is selectively permeable to hydrogen . I n practice, useful metals for this purpose are steel/iron, and palladium. For many purposes, the metal membrane is made of a sheet of steel/iron which is coated with palladium on one or both sides. The thickness of the membrane determines the measuring response time of the device. A palladium membrane or coating is normally in the range of 0.05-10 μm, preferably 0.1 -0.5 μm, while a steel membrane may be considerably thicker, typically from about 20 μm to about 1 mm.

The metal/metal oxide cathode used according to the present invention should be of a kind which is able to oxidize hydrogen into hydrogen ions, but which will not give rise to any undesired oxidation reactions with other components present. Thus, preferred cathodes used accor¬ ding to the invention have an oxidation potential which is between 200

f OMPI and 1200 mV above the equilibrium potential of the hydrogen electrode at the pH of the electrolyte solution , in particular 600- 1200 mV above the equilibrium potential of the hydrogen electrode at the pH of the electrolyte solution . I n the embodiments preferred at present, the oxidation potential of the cathode is about 900 mV above the equili¬ brium potential of the hydrogen eletrode at the pH of the electrolyte solution .

Subject to this, the metal oxide of the cathode may be selected from, e.g. , silver oxide, mercury oxide, and manganese dioxide. The metal of the metal/metal oxide cathode only serves as an electrical conduc¬ tor, but should be of a kind which is substantially chemically inert in relation to the metal oxide and the electrolyte solution . Hence, the metal may suitably be selected from nickel , nickel-coated steel , silver, and noble metals such as gold, palladium, and platinum.

The potential of the metal/metal oxide cathode may also be expressed with .reference to the range required for anodic oxidation of hydrogen on the metal membrane, preferably in the range of -100 to ⁺300 mV

The cathode should be designed so that it is able to supply cu rrent to the metal membrane in sufficient quantity to oxidize all hydrogen passing th rough the membrane under normal conditions of application, while still remaining in the acceptable potential range.

Also, the total capacity of the cathode should preferably exceed the normal useful life of the metal membrane.

The electrolyte solution used according to the invention and communi¬ cating with the metal membrane and the metal oxide of the cathode should be an electrolyte which is suitably adapted to the type of the metal oxide. As examples of electrolyte solution may be mentioned aqueous alkaline solutions such as sodium or potassium hydroxide, normally in concentrations in the range of 0. 1 -14N . For some appli¬ cations, e. g . when the measurement is to be performed at high tem¬ peratu res , it may be preferred to use an acidic electrolyte solution in combination with an appropriate metal oxide, e. g. , sulphuric acid in combination with lead oxide.

In one embodiment of the device according to the invention, the metal membrane is supported on a plastic foil . I n this embodiment, the metal membrane may be a steel/iron membrane having a palladium coating on one or both sides, or may be constituted by a palladium coating on the piastre foil . I n this embodiment, the plastic foil may advantageous¬ ly be arranged so as to constitute an outer face of the device pro¬ viding a mechanical protection of the metal membrane, and in order to provide access for hydrogen to the metal membrane, the plastic foil may be perforated or may be made of a hydrogen-permeable plastic material . When providing a perforation of the supporting plastic foil , any plastic material which is not affected by the actual corrosive forces may be employed, such as a foil made of e. g. Mylar®. Alter- natively, the hydrogen permeable plastic foil may be made of, e. g . , polytetraflouroethylene (Teflon®) .

In this embodiment of the device according to the invention , the metal/metal oxide cathode may also be supported on a plastic foil, preferably a hydrogen-impermeable plastic foil, constituting a wall portion of the housing . The metal/metal oxide cathode supporting plastic foil is preferably made of a strong, inelastic and pliable plastic material, such as Mylar®. When providing the metal membrane and the metal/metal oxide cathode supported on respective plastic foils, the supporting foils may be kept in a spaced-apart relationship by means of a separator and be joined and sealed to one another along an outer rim portion . I n this embodiment, a so-called "Patch Cell" is provided which is adapted to be mounted on an outer surface of a steel or iron wall, the corrosive influences on which are to be determined. I n this so-called " Patch Cell" , the electrolyte solution may be a layer of an electrolyte paste which is arranged on the side of the metal membrane opposite to the hydrogen entry side, and, consequently, concealed within the space defined between the metal membrane supporting foil and the metal/metal oxide cathode supporting foil . I n a further embodiment of the invention , the device comprises a first device part and a second device part, both having any of the above characteristics, wherein the first device part constitutes an active sensing element, and the second device part, which is sealed so as to exclude hydrogen , constitutes a reference element. I n this embodi¬ ment, the second device part is adapted to respond to any environ¬ mental influences other than the corrosive forces or influences which are being measured by means of the first device part. Thus, the second device part may constitute a temperature responsive sensor. When connected to appropriate measuring equipment, environmental influences other than the corrosive influences which are to be deter¬ mined by means of the measu ring results provided by the fi rst device part, may be compensated for by means of measu ring results provided by the second device part. Such envi ronmental influences other than corrosive influences may be e. g . temperatu re variations .

I n an important embodiment of the method according to the invention , the hydrogen flux is generated as a result of a chemical or electro¬ chemical reaction between the entry su rface of the membrane and an exterior fluid medium such as a liquid or gas with which the surface of the membrane reacts chemically or electrochemically. This makes it possible to use the membrane material as a model for any object made of the same material and subjected to chemical or electrochemical reaction, so that the hydrogen flux measured by the method of the invention will be representative of the hydrogen flux to which the object made of the same material will be subjected when exposed to the environment in question .

Thus, in one embodiment, the entry side of the membrane is exposed to a fluid which corrodes the surface metal of the membrane with generation of a hydrogen flux into the membrane. An important exam- pie of this is where the membrane is made of a sample of a steel type, the hydrogen corrosion of which, when subjected to chemical or electrochemical treatments in a cleaning solution , in pickling, phos- phating, stripping or metalplating baths , or in other electrogalvanic processes , is to be judged . The membrane may also be made of steel being subjected to corrosion in a process stream of a chemical plant or a power plant, or when measuring or processing natural gas, oil, or geothermal steam.

It is also possible to provide the membrane with an outer protective coating or layer, the protective effect of which is to be studied when subjected to corrosive or other influences . Alternatively, the metal membrane may be provided with an outer coating or layer of a mate¬ rial, the corrosive influence of which on the metal membrane is to be studied.

Also, the method of the invention may be utilized for determining hydrogen uptake in steel subjected to corrosion in natural environ¬ ments, ranging from atmospheric exposure to natural waters and soils, including exposure to or in building materials, such as con¬ crete, thermally insulating materials, wood, etc.

According to the invention, the metal membrane may be exposed to the said environments or materials, also including the case where solid materials are applied to the membrane, and the case where the device is built into or applied onto a material the hydrogen influence of which on the steel is to be monitored.

The membrane may be provided with surface coatings corresponding to the simulation desired, including chemically corrosive layers, paints, metal coatings, anticorrosive protective layers, including paints, etc. , the hydrogen uptake of which or the influence of which on the hydrogen uptake of the membrane metal is to be investigated or monitored .

The size and shape of the membrane exposed may be adapted to suit the purpose in question , e.g . , to simulate the functions of an engi¬ neering component.

The membrane may also be subjected to additional electrochemical control or bimetallic contact in order to make it possible to study the effect of cathodic protection , galvanic action, or any other means of control .

OMPI Furthermore, the method of the invention may be used for controlling (testing and/or standardizing) test solutions in which stress corrosion or hydrogen embrittlement tests are carried out.

A very important example of determination of hydrogen uptake in steel or iron subjected to corrosion in natu ral environments is the determination of hydrogen uptake in a steel or iron wall of a pipe or vessel . I n this case, the metal membrane is arranged adjacent to the outer su rface of the steel or iron wall , and the flux of hydrogen escaping from an outer surface of the steel or iron wall is deter- mined.

BRI EF DESCRI PTION OF THE DRAWI NG

The invention will now be fu rther described with reference to the drawing, wherein

Fig . 1 is a vertical , sectional view of a fi rst embodiment of a hy- drogen probe or hydrogen monitoring cell according to the invention,

Fig . 2 is an illustration of a possible application of the hydrogen monitoring cell shown in Fig, 1 ,

Fig . 3 is a vertical , sectional view of a prototype of a measuring cell constructed in accordance with the principles of the present inven- tion,

Fig . 4 is a diagramme showing the current plotted versus time in a test application of the prototype shown i Fig . 3,

Fig . 5 is a partly sectional view of a second embodiment of a hydro¬ gen probe or hydrogen monitoring cell according to the invention, Fig . 6 is a partly sectional view of a third embodiment of a hydrogen probe or hydrogen monitoring cell according to the invention , Fig . 7 is a partly sectional view of a fou rth embodiment of a hydro¬ gen probe or hydrogen monitoring cell according to the invention or a "PATCH CELL" , Fig . 8 is an exploded view of the embodiment or " PATCH CELL" shown in Fig . 7, and

Fig . 9 is a partly sectional view of a fifth embodiment of a hydrogen probe or hydrogen monitoring cell according to the invention . DETAI LED DESCRI PTION OF THE DRAWING

Fig. 1 shows a first embodiment of a hydrogen monitoring cell ac¬ cording to the invention designated 10 in its entity. The cell 10 is constructed as a button -shaped power cell (a mercu ry battery) of the kind commonly used in watches, pocket computers, hearing aids or the like. The cell 10 comprises two main components, a bottom part and a top part designated 11 and 12, respectively. The top part 12 constitutes a membrane wall of the measuring cell and is made from steel sheet comprising pu re iron or steel plate of the kind commonly used for fabrication of tϊnplate in the canning industry. The interior surface of the top part 12, i . e. the surface facing the interior of the measuring cell , is provided with a coating of palladium. The bottom part 11 , together with a metal oxide filling 14, constitutes a metal/- metal oxide cathode of the hydrogen monitoring cell . The metal/metal oxide cathode may be constructed in several ways using several combinations of metals and oxides, all of which are known from the construction of alkaline batteries . The bottom part 11 is made of a metal which is not or only very slowly oxidized in a solution at the potential of the cathode, such as nickel or nickel-coated steel, stain- less steel, or a noble metal such as silver, gold or palladium or a coating of these metals on steel or another suitable carrier. The metal oxide may be manganese dioxide, silver oxide, mercury oxide, and other oxides either alone or in combination with metal powders, mer¬ cury or other additives serving to give conductivity, to increase the effective area or to improve the performance of the cathode in other ways known from the construction of cathodes in alkaline power generating cells . The metal oxide filling 14 is press-fitted into the bottom part 11 in order to obtain good contact between the metal and the metal oxide.

The two components, i . e. the bottom part 11 and the top part 12 are sealed together, a sealing compound 15 being provided within the in-turned edge of the top part 12. The thickness of the top part 12 is mainly determined by the ability of the material to be formed in the turned-in edge. However, the thickness of the material also deter- mines the useful life of the measuring cell and the response time

ϋ£5 thereof. As will be appreciated, a more heavy metal sheet provides a longer useful life time, but reduces the response time of the moni¬ toring cell . A sheet thickness of 0.25 mm commonly used in the can¬ ning industry provides satisfactory results relating to these above mentioned factors . The thickness of the palladium coating of the interior su rface of the top part 12 may be within the range of 0.05 to 10 μ , preferably 0. 1 to 0.5 μm.

Within the hydrogen monitoring cell 10, in a space defined between the palladium coating 13 and the upper surface of the metal oxide filling 14, an electrolyte 16 and a separator 17 are provided. The electrolyte 16 may be a pu re solution of al kali hydroxide, preferably sodium or potassium hydroxide in a concentration of 0. 1 to 14 molar, preferably 1 molar. The separator 17 serves to prevent particles of metal oxide from getting into contact with the palladium coating and may be made of polymer fibres, e. g . polypropylene fibres commonly used as separators in alkaline batteries .

I n Fig . 2 the fi rst embodiment of the hydrogen probe or hydrogen monitoring cell is shown mounted at the end of a tube 18 and in electrically conductive connection therewith . The tube 18 may be made of, e. g . , steel or iron and is provided with an interior flange part 19. The bottom part 11 of the measuring cell 10 is insulated relative to the tube 18 by means of a insulating annular sealing 20 which is mounted in the interspace between the interior surface of the tube 18 and the exterior surface of the bottom part 11 . I n a central bore 21 of the flanged part 19, a spring contact 22 is mounted in an insu¬ lating sealing 23. Through a soldered joint 24, the contact 22 is connected to an insulated wire 25 for establishing con nection to ex¬ ternal cu rrent measu ring equipment also connected to the tube 18 (not shown) . Surrounding the tube 18, a corrosive resistant tube 26 having a tu rned-in flange 27 is mounted . The tube 26 is insulated relative to the measuring cell 10 by means of an insulating annular sealing 28 which is mounted between the flange 27 and the top sur¬ face of the membrane 12 of the measu ring cell . Fu rthermore, an insulating sealing (not shown) may be provided within the space between the two tubes 18 and 26 in order to keep the tubes in spaced-apart relationship. The measuring cell mounted at the end of the tube 18 may thus be introduced or withdrawn from a measuring environment in a manner known per se through one or more ball valves.

In Fig . 3 a prototype constructed in accordance with the principles of the present invention is shown . The prototype is designated 30 in its entity and comprises a housing consisting of a bottom component 31 and a top component 32 made of polymethacrylate. The two compo¬ nents 31 and 32 are joined together by means of screw connections 33 and 34 and confine a membrane 35 within the interspace between the two components . The membrane 35 is made of a piece of detinned and finely polished tin plate with a thickness of 0.25 mm and is coated with palladium on one side, the side facing the interior of the bottom component 31 . The palladium coating is deposited from the following solution :

per liter: 5 g PdC

240 g Na-PO ,, aq

55 g ( H₄)₃PO₄

3.5 g benzoid acid, pH adjusted to 11 with ammonia .

Working conditions : Temperature 60-70°C, current density 2 mA/cm¹, time 2-4 minutes.

Thus, a palladium coating of a thickness of approximately 0.1 -0.2 μm is obtained . The opposite, uncoated side of the steel membrane pro- vides an exposed area of 7 cm² within a central circular recess of the top component 32. The membrane 35 is secured and sealed relative to the interior of the bottom part 31 by means of an annular -sealing 37 mounted in an annular recess 38 provided in the top surface of the bottom component 31 . I n the interior of the bottom component 31 , two co-axically arranged cavities or chambers 39 and 40, respectively, are provided . The chamber 40 is filled with a metal/metal oxide electrode 41 which is connected to an insulated wire leading th rough a central bore 43 of the bottom component 31 . The metal/metal oxide electrode

O^H_- 41 is secured in the chamber 40 by means of a sealing 44 also sealing the chamber 39 relative to the environment. The sealings 37 and 44 are made of alkali resistent rubber. The metal/metal oxide electrode is constituted of a nickel-coated steel shell which is filled with a mixture of equal weights of mercury oxide and a powder of metallic silver. The powders are pressed into the shell . The wire 42 is soldered to the exterior su rface or bottom su rface of the nickel-coated steel shell . Within the chamber 39 a 1 N sodium hydroxide electrolyte solution of high purity is confined .

When assembling the cell 30, the cell is flushed with pu re nitrogen and then placed in a de-aerated electrolyte solution . While immersing the cell in the electrolyte solution , vacuum is applied, which helps to remove gasses before closing the cell .

Five cells of the kind shown - in Fig . 3 have been constructed in accordance with the principles described above. They were stored for 24 hours in a short-circuited state. Having been short-ci rcuited for 24 hours, the cells generated cu rrents in the range 0. 1 -0.5 μA, ty¬ pically 0.2 μA. Since the internal area of the membrane is 16- cm², the currents thus obtained equal a current density of approximately 0.01 -0.03 μA/cm².

I n Fig. 4 a diagramme of current generated in a prototype cell of the kind shown i Fig . 3 is shown plotted versus time. The zero current (not shown in Fig . 4) of the cell is approximately 0.2 μA as men¬ tioned above. At the time zero, the cell is immersed In a 0.2 molar H SO₄ solution . Within approximately 7 minutes the cu rrent generated in the cell rises to a value (125 μA) of approximately 50% of the maximum current (250 μA) . After having stabilized at the maximum cu rrent, the cell is removed from the solution at the time indicated by an arrow in Fig . 4. The cu rrect decays fai rly rapidly . However, the transient response of the measu ring cell may be increased (or de¬ creased) by decreasing (or increasing) the thickness of the mem¬ brane. Responses identical to the one shown in Fig . 4 were obtained in three successive measu rements . Thus, the cell does not exhibit hysteresis .

OMPI I n Fig\ 5, a second embodiment of the hydrogen measuring probe or hydrogen detecting cell is shown designated 50 in its entity. The hydrogen measuring probe 50 comprises a sensing element, i . e. a membrane, of a short length of a thin-walled steel or iron tube 51 closed at one end 51a . The interior su rface of the tube 51 may be provided with a palladium coating although this is not mandatory to the proper functioning of the probe. The probe also includes a metal/metal oxide electrode comprising a rod of a metal/metal oxide powder mixture 52 which may be of the kind described above in connection with Fig . .3 and which is pressed around a metal conductor 53 of e. g. nickel . A separator 54 is provided surrounding the metal/metal oxide electrode 52, 53. The separator may be made of e.g. propylene fibres. The assembly of metal/metal oxide electrode 52, 53 and separator 54 is fitted into a plastic stopper 55 which is press-fitted into the open end of the tube 51 after filling the interior space of the tube with a de-aerated electrolyte solution 56 which may be of the kind described above in connection with Fig . 3. The metal conductor 53 projects through the stopper 55 and is connected to an insulated wire 57. The tube 54 is also connected to an insulated wire 58. The wires 57 and 58 are adapted to be connected to external current measuring equipment, e.g . a conventional sensitive am-meter. A plastic jacket 59 is heat-sh rinked around the o^*pen end of the tube 51 and around the wires 57 and 58.

In Fig. 6, a third embodiment of the invention is shown . The embodi- ment of the invention shown in Fig. 6 and designated 60 in its entity differs only slightly from the embodiment of the invention shown in

Fig . 5. Thus, identical reference numerals are designating identical components of the two embodiments . As distinct from the probe 50, the probe 60 comprises a tube 61 of a metal impermeable to hydrogen, e.g . nickel , iron/nickel alloy or stainless steel . The inside of the tube 61 may be covered with an alkali-resistant enamel . The end of the tube 61 opposite to the end fitted with the plastic stopper 55 is provided with a steel or i ron disc which is welded to the tube 61 .

The disc 62 constitutes the metal membrane of the hydrogen measur- ing probe in accordance with the principles of the present invention .

The embodiments of the invention shown in Fig . 5 and Fig . 6 are intended and adapted to be used when measuring in situ, e. g . when the metal is subjected to an electrogalvanic process . Thus, the probe is dipped into the electrogalvanic solution to simulate the process normally performed in the bath (e. g . cleaning, pickling, phospating, stripping, electroplating, electrodeposition etc. ) while the tube of the hydrogen probe, i . e. the tube 51 of the embodiment shown in Fig . 5 and the tube 61 of the embodiment shown in Fig . 6, respectively, is connected to external electrogalvanic current/voltage generating means.

The long and slender shape of the hydrogen probe 60 shown in Fig . 6 provides additional useful properties . Thus, the shape of the hy¬ drogen probe makes it easily adaptable to an "access fitting" . Fur¬ thermore, the length of the hydrogen probe when combined with a tube with low thermal conductivity, e. g . a stainless steel or iron/- nickel alloy tube, makes it possible to apply cooling to the end of the tube which is provided with the plastic stopper 55, while maintaining the end of the tube which is provided with the metal disc 62 at a significantly higher temperature in the galvanic bath . Th us the tem¬ perature dependency of the measurement is reduced . The combination of a hydrogen-impermeable metal tube 61 and hydrogen-permeable metal disc 62 renders it possible to perform measu rements on special, possibly user-supplied iron or steel materials .

In Figs. 7 and 8, a fourth embodiment of the hydrogen measuring device or a so-called " PATCH CELL" is shown . The "PATCH CELL" shown in Figs . 7 and 8 is designated 70 and adapted to be mounted on an outer surface of a steel or iron wall which is exposed to cor¬ rosion . Thus, the "PATCH CELL" is adapted to detect and measure hydrogen permeating th rough the steel wall and liberated from the outer su rface of the iron or steel wall within a measu ring area deter- mined by the physical dimensions of the measu ring device. Fig . 7 is a vertical sectional view of the measu ring device 70 when assembled and ready for use, while Fig . 8 is an exploded view thereof. Thus, iden¬ tical reference numerals designate identical parts in Figs . 7 and 8. The measuring device 70 comprises an outer protective foil 71 of a suitable plastic material which is pliable, strong and inelastic, e. g . Mylar®.. A thin metal layer 72 is firmly bonded to the plastic foil 71 and constitutes a metal component of a metal/metal oxide electrode of the measuring device. The metal layer 72 may be made from any appropriate metal, e.g. nickel or silver. Apart from supporting the metal layer 72, the plastic foil 71 provides protection and insulation to the metal layer relative to the environment. The metal layer may be deposited as a thin metal coating on the plastic foil 71 . The metal oxide component of the metal/metal oxide electrode is designated by the reference numeral 73. The metal oxide may be made from a mix- ture of metal oxide powder and metal powder which are pressed into an intimate mixture also containing a binder if desired. The metal oxide component is applied to the metal component of the metal/metal oxide electrode, i .e. the metallic layer 72, e. g. by thick film tech¬ nique.

Similar to the above-described embodiments of the invention the measuring device 70 comprises a metal membrane which is constituted by a foil 77 of a metal permeable to hydrogen , e. g. palladium or iron coated with palladium on both sides . Similar to the combination of the plastic foil 71 and the metallic layer 72, the metal foil or membrane 77 is firmly bonded to a supporting plastic foil 78. However, the plastic foil 78 is provided with several holes 79 in order to provide external access to the membrane 77. Alternatively, the plastic foil may be made of a material which is permeable to hydrogen, e. g . polytetrafluoro- ethylene (Teflon®) .

The metal oxide 73 of the metal/metal oxide electrode is prevented from coming into contact with the metal membrane 78 by means of a separator 74. The separator 74 may be made from a sheet of alkali resistant filter paper, e. g . made from polypropylene fibers . The rim of the separator 74 is on both sides provided with layers 75 and 76 of a plastic material which permeates the filter material and also func¬ tions as a glue and sealing compound when the measu ring device is assembled in a heat-sealing process to be described in further detail below. The material of the layers 75 and 76 must fulfill two functions, firstly, the material must be able to permeate and fill the pores of the separator and, secondly, the material must have adhesive properties in order to firmly join the metallic layer 72 and the metal membrane 79 together and seal the measuring device.

As will be appreciated, the plastic foil 71 , the metallic layer 72 and the metal oxide component 73 are prepared as one single unit to be joined and sealed with another unit comprising the metal membrane 77 and the plastic foil 78 by means of the adhesive rim material 75 and 76 of the separator 74. Before assembling the measuring device, an electrolyte solution is provided between the metal/metal oxide elec¬ trode and the metal membrane 77. The electrolyte solution is a de- aerated al kaline electrolyte preferably made into a paste using tech¬ niques known from the manufactu ring of batteries . The metal oxide component 73 and the separator sheet 74 are soaked with electrolyte solution and, fu rthermore, a layer of the electrolyte solution is ap¬ plied on the metal membrane 77. When assembling the cell , air (oxy- gen) must be excluded from the interspace between the metal/metal oxide electrode and the metal membrane. Fu rthermore, the electrolyte solution must be prevented from coming into contact with the adhesive material 75 and 76. The cell is assembled by means of an appropri¬ ately heated tool comprising two parts which are contacted with the rim portions of the plastic foil 71 and 78, respectively, and pressed together.

The plastic foil 71 , the metallic layer 72, the metal membrane 77 and the plastic foil 78 are provided with flaps 71a, 72a, 77a, and 78a, respectively. The metal flap 72a is intended to be folded around a metal wire 82 and the metal flap 77a is adapted to be folded around a metal wire 83, as shown in Fig . 7. Thus, the plastic flap 71 a and the plastic flap 78a insulate the metal flaps relative to one another and relative to the environment. The metal wires 82 and 83 are intended to be short-ci rcuited when the measu ring device is stored, and to be connected to external current measu ring equipment such as a con¬ ventional am-meter when in use.

Finally, a cover made of paper or cardboard is applied on the outer su rface of the plastic foil 78 by means of a compressible, flexible material 80 such as foam rubber or foam plastic with self-adhesive

OMPI surfaces in order to protect the exposed surface areas of the metal membrane 79 when the measu ring cell is not in use. The adhesive surface of the material 80 facing the cover 81 also serves to apply the measuring device to a measuring site on a steel or iron wall .

In Fig . 9 a fifth embodiment of the hydrogen measu ring device ac¬ cording to the invention is shown and designated 90 in its entity. The embodiment of the invention shown in Fig . 9 differs from the above described embodiments in that the device comprises two ba¬ sically identical parts or halves . The first part constitutes the active sensing element, and the second part is neutralized or inactive or adapted to be neutralized or inactivated so that the second part constitutes a reference element. Unlike the fi rst part of the measuring device, the second part or reference element is not intended to be exposed to corrosive forces or other influences to be measured . Instead, the reference element responds to temperature variations only. When connected to appropriately calibrated differential measur¬ ing equipment, the first part of the measu ring device provides cur¬ rent measurements while the second part or reference element serve temperature compensation purposes when supplying current to the measuring equipment.

The measuring device 90 comprises a metal sheet 91 , e.g. a nickel or silver sheet. The metal sheet 91 is common to both the above de¬ scribed parts of the measuring device and provides support thereto and physical strength to the measuring device as a whole. On a first side of the metal sheet on which the active sensing part of the meas¬ uring device is constructed, a metal oxide component 92 is applied in intimate contact with the metal sheet 91 . Together, the metal sheet 91 and the metal oxide component 92 constitute a first metal/metal oxide electrode of the active part of the measuring- device. Above the metal oxide component 92, a separator 93 is arranged . Above the separator 93 a metal membrane 94 is arranged . Furthermore, the first part of the measuring device comprises an electrolyte solution which is en¬ closed in the space between the metal/metal oxide electrode and the metal membrane 94. Basically, the above described components of the measuring device are identical to the corresponding components of the

OMPI embodiment of the invention shown in Figs . 7 and 8. Thus, a hydro¬ gen permeable plastic foil serving protective purposes may also be arranged so that it covers the metal membrane 94.

The second part of the measuring device comprises components ba- sically identical to the above described components, i . e. a metal oxide component 95, a separator 96, a metal membrane 97, and an elec¬ trolyte solution enclosed in the space between the metal/metal oxide electrode and the metal membrane 97. Furthermore, the second part of the measu ring device 90 comprises a corrosion resistant foil 98 which completely seals the second part of the measuring device and protects the metal membrane 97 when the measu ring device is subjected to corrosive forces . The foil 98 may either constitute an integral part of the measuring device or be applied by a user. Thus, the foil 98 may be made of plastic or may be applied as an anticorrosive composition, paste or the li ke.

The entire measu ring device 90 is assembled in a heat-sealing process similar to the process described above in connection with Figs . 7 and 8.

The metal sheet 91 , the metal membrane 94, and the metal membrane 98 are connected to individual wires 99, 100, 101 , respectively, and the wires 99-101 are enclosed within a common jacket 102.

It is understood that the above described embodiments are merely il¬ lustrative of the application of the principles of the present inven¬ tion . Numerous other embodiments may be devised by those skilled in the art without departing from the spirit and scope of the present in¬ vention .

— OMPI REFERENCES

1 . M.A.V. Devanathan and Z. Stachu rskϊ: Proceeding of the Royal Society, 1962, Vol A270, p90.

2. M. Meron et al, Metal Progress, July 1981 , p.52.

3. GB Patent Specification No. 1 585 070

4. GB Patent Specification No. 1 524 017

5. G . Mansfeld, S . Jeanjaquet, and D. K. Roe, Materials Performance, February 1982.

Claims

1 . A device for determining hydrogen flux through a metal membrane, which is selectively permeable to hydrogen, from a hydrogen entry side, comprising the metal membrane, an electrolyte solution commu- nicating electrolytically with the side of the membrane opposite to the hydrogen entry side, a metal/metal oxide cathode communicating electrolytically with the electrolyte, the cathode being capable of oxidizing hydrogen into hydrogen ions, the electrolyte and the catho¬ de being contained in a housing of which the metal membrane consti- tutes a wall portion , and leads for electrical connection to external current measuring equipment, the device being adapted to be main¬ tained in a substantially short-ci rcuited condition by direct connection between the leads or by connection to a cu rrent measuring system with low internal impedance.

2. A device for determining hydrogen flux through a metal membrane, which is selectively permeable to hydrogen , from a hydrogen entry side, comprising the metal membrane, an electrolyte solution communi¬ cating electrolytically with the side of the membrane opposite to the hydrogen entry side, and a metal/metal oxide cathode communicating electrolytically with the electrolyte, the cathode being capable of oxidizing hydrogen into hydrogen ions, the electrolyte and the catho¬ de being contained in a housing of which the membrane constitutes a wall portion , the housing being permanently sealed .

3. A device according to claim 1 or 2 wherein the membrane comprises iron or steel with a palladium coating on one or both sides which has a thickness within the range of 0. 1 -10 μm, preferably 0.1 -5 μm.

4. A device according to any of claims 1 -3 wherein the cathode has an oxidation potential which is between 200 and 1200 mV above the equilibrium potential of the hydrogen electrode at the pH of the elec- trolyte solution .

5. A device according to claim 4 wherein the oxidation potential of the cathode is 600-1200 mV above the equilibrium potential of the hydro¬ gen electrode at the pH of the electrolyte solution .

6. A device according to claim 4 wherein the oxidation potential of the cathode is about 900 mV above the equilibrium potential of the hydro¬ gen electrode at the pH of the electrolyte solution .

7. A device according to any of claims 1 -6 wherein the metal oxide of the cathode is selected from silver oxide, mercury oxide, and man¬ ganese dioxide, and the metal of the cathode is selected from nickel, nickel-coated steel , silver, and noble metals such as gold, palladium, and platinum.

8. A device according to claim 7 wherein the electrolyte is a de-aera- ted alkaline solution such as sodium or potassium hydroxide having a concentration of 0. 1 -10M.

9. A device according to any of the preceding claims wherein the metal membrane is supported on a plastic foil .

10. A device according to claim 9, wherein the plastic foil constitutes an outer face of the device and is perforated in order to provide hydrogen access to the metal membrane.

11. A device according to claim 9, wherein the plastic foil constitutes an outer face of the device and is made of a hydrogen permeable plastic material .

12. A device according to any . of the preceding claims wherein the metal/metal oxide cathode is supported on a hydrogen-impermeable plastic foil constituting a wall portion of the housing .

13. A device according to any of the claims 9-11 and 12 wherein the metal membrane supporting foil and the metal/metal oxide cathode supporting foil are kept in spaced-apart relationship by means of a separator and joined and sealed to one another along an outer rim portion .

14. A device according to claim 13 wherein the electrolyte solution is a layer of an electrolyte paste which is arranged on the side of the metal membrane opposite to the hydrogen entry side.

OMPI

15. A- device according to any of the preceding claims comprising a first device part and a second device part both having any of the above characteristics, wherein the first device part constitutes an active sensing element, and the second device part, which is sealed so as to exclude hydrogen, constitutes a reference element.

16. A method for determining hydrogen flux through a metal mem¬ brane, which is selectively permeable to hydrogen , from a hydrogen entry side, comprising: providing a housing, arranging a metal/metal oxide cathode and an electrolyte solution in the housing, which cathode is capable of oxid¬ izing hydrogen into hydrogen ions and communicates electrolytically with the electrolyte solution, arranging in the housing the metal membrane so that it constitutes a wall portion thereof, contacting the side of the metal membrane opposite to the hydrogen entry side with the electrolyte solution, sealing the metal membrane in the housing, short-circuiting the metal membrane and the metal/metal oxide cath¬ ode, and measuring the electrical current generated as a result of oxidation of hydrogen passing th rough the metal membrane.

17. A method for determining hydrogen flux through a metal mem- brane, which is selectively permeable to hydrogen , from a hydrogen entry side, comprising : providing a housing, a rranging a metal/metal oxide cathode and an electrolyte solution in the housing, which cathode is capable of oxid¬ izing hydrogen into hydrogen ions and communicates electrolytically with the electrolyte solution, arranging the housing in contact with the metal membrane so that at least part of the metal membrane con¬ stitutes a wall portion of the housing, contacting the side of the metal membrane opposite to the hydrogen entry side with the electrolyte solution, sealing said part of the metal membrane relative to the housing, short-circuiting the metal membrane and the metal/metal oxide cathode, and continuously measu ring the electrical cu rrent generated as a result of short-ci rcuiting the metal membrane and the metal/metal oxide cathode.

18. A method according to claim 16 or 17 wherein the hydrogen flux is generated as a result of a chemical or electrochemical reaction between the entry surface of the membrane and an exterior fluid medium such as a liquid or gas with which the surface of the membrane reacts chemically or electrochemically.

19. A method according to claim 18 wherein the entry side of the membrane is exposed to a fluid which corrodes the surface metal of the membrane with generation of a hydrogen flux into the membrane.

20. A method according to claim 18 or 19 wherein the membrane is made of a sample of a steel type, the hydrogen corrosion of which, when subjected to chemical or electrochemical treatments in a cleaning solution, in pickling, phosphating, stripping or metalplating baths, or in other electrogalvanic processes, is to be judged.

21 . A method according to claim 18 or 19 wherein the membrane is made of steel being subjected to corrosion in a process stream of a chemical plant or a power plant or when measuring or processing natural gas, oil, or geothermal steam.

22. A method according to any of the claims 18-21 wherein the metal membrane is provided with an outer protective coating or layer, the protective effect of which is to be studied when subjected to corro- sive or other influences.

23. A method according to any of the claims 18-21 wherein the metal membrane is provided with an outer coating or layer of a material, the corrosive influence of which on the metal membrane, is to be studied.

24. A method according to claim 16 or 17 wherein the flux of hydrogen escaping from an outer su rface of a steel or iron wall of a pipe or vessel is determined, and wherein the metal membrane is arranged adjacent to the outer surface of said steel or iron wall .