NO833886L - METHOD AND DEVICE FOR DETERMINING HYDROGEN FLOW. - Google Patents

Description

The present invention relates to a method and

a device for determining hydrogen flow through a metal membrane.

Technical background

In some chemical process industries, steel components can corrode under conditions that give rise to hydrogen

induced cracking or blistering. A typical and technically important example is corrosion of lines and pressure 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 during evolution of hydrogen. The presence of hydrogen sulphide inhibits the formation of molecular hydrogen, and a significant part of the hydrogen penetrates into the steel, where it can cause damage in the form of cracking or blistering.

It is possible to reduce the hydrogen damage in several ways, for example by avoiding condensation, by removing hydrogen sulphide or by adding inhibitors that can perform their function in several ways, for example by neutralizing the liquid phase or by forming a protective film .

With a view to being able to monitor the effect of these precautions, the hydrogen uptake or the rate of hydrogen uptake in an exposed area of the relevant steel component should be measured. This can be done according to several known measuring principles, most of which are based on the fact that the hydrogen penetrating the exposed entrance side of the steel wall will diffuse through the steel wall and be released in gaseous (molecular) form on the side of the steel wall that is opposite the entrance side. The rate of hydrogen evolution on the aforementioned opposite side of the steel wall can be measured using known techniques, for example by measuring the development of pressure in a closed cavity or by measuring the displacement of a liquid drop in a glass tube that has a calibrated inner wall. The amount of hydrogen can also be analyzed in different ways, for example by gas phase chromatography.

For carrying out the measurement of the rate of hydrogen evolution according to the above physical measurements

principles, several hydrogen measurement probes have been proposed.

However, these hydrogen measurement probes suffer from various disadvantages. Some of them are rather primitive and have low sensitivity, while others need constant supervision and are not suitable for automated data registration. Others are more sensitive and meticulous, but are quite expensive and delicate.

An electrochemical measuring principle based on the electrochemical oxidation of hydrogen that penetrates to the side of the steel wall opposite the entrance side is also known.

This technique was first described 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 plate forms a wall or membrane between two chambers. A first chamber constitutes an experimental chamber in which the iron or steel material in 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 chamber contains a degassed alkaline solution, for example one molar potassium hydroxide. The surface of the iron or steel wall facing the second chamber is covered with a thin coating of electrodeposited palladium, which is permeable to hydrogen and stable in the alkaline solution. A platinum counter electrode is arranged opposite the iron or steel wall, and a reference electrode is also arranged in the second chamber with its end arranged quite close to the palladium-covered membrane. To maintain a constant potential on the palladium-coated iron or steel wall in the alkaline solution, a potentiostat is used. In general, a potential of EH= +100 mV is used. At this potential, the palladium coating and the iron or steel material in the wall are passive. The passive current density is therefore quite small, typically between 0.01 and 0.1 yA per cm 2. The hydrogen that had to diffuse through the iron or steel wall and its palladium coating is oxidized on the side of the palladium coating that is opposite the entrance side of the iron or steel wall, according to the following equation:

Consequently, an increase in anodic current beyond the passive background current is equivalent to a hydrogen flow through the wall. The hydrogen flow through a unit area of the iron or steel wall of a given thickness is a product of the hydrogen activity on the outer side of the wall and the diffusion admittance through the wall, provided that the potential is able to maintain a hydrogen activity at approx. zero on the opposite side, i.e. the inner palladium-coated side

off the wall. If the diffusion admittance is known, the hydrogen activity in the first chamber can be calculated on the basis of the anodic current in the cell. The diffusion admittance is determined by using a regulated electrochemical environment in the first chamber or by introducing a hydrogen atmosphere of known pressure into the first chamber, the outer side of the iron or steel wall being coated with palladium, which makes the cell sensitive to molecular hydrogen.

Another laboratory measuring device based on the Devanathan measuring principle is described in Reference 2. In this measuring device, a steel plate forms part of a wall in a cell containing an electrolyte solution, a reference electrode and a counter electrode. Furthermore, a potentiostat is needed for operation of the device.

GB Patent No. 1 585 070 describes an electrochemical cell for determining the concentration of hydrogen in a fluid, which electrochemical cell is based on the principles of the Devanathan measurement technique. The cell comprises a container for an electrolyte solution, a working electrode which forms a wall part of the container and is arranged to be exposed to a fluid under test, and a platinum/platinum oxide reference electrode which has an area comparable to the working electrode and is arranged close to and parallel to the working electrode. In one embodiment of the cell, a counter electrode, 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 must be used in combination with electronic means to keep the working electrode at a fixed potential. In another embodiment of the measuring cell, the electronic means, the counter electrode and the current-determining means are omitted. Instead, the working electrode and the reference electrode are connected by potential-measuring means. According to what is disclosed in GB Patent No. 1,385,070, a potential difference between the electrodes is related to the concentration of hydrogen at the working electrode. However, the patent description explains that a stable reading can only be obtained with low hydrogen activities.

Another hydrogen measuring cell based on the same measuring principles is known from GB patent no. 1 524 017. The cell (a so-called "Patch cell") is designed to be fixed in fluid-tight connection with an outer surface of a steel or iron wall, the inner side of which is exposed to corrosion. The cell reacts quantitatively to all hydrogen escaping from the outer surface of the steel or iron wall, so that the amount of hydrogen formed on the inner surface of the steel or iron wall is measured.

In an article by F. Mansfeld et al. (reference 5) describes a measurement system based on the Devanathan measurement principle described above. The system comprises a measuring cell consisting of a cell body which is pressure-fitted in a cylindrical magnet device which serves the purpose of fixation when placed in contact with a metal sample body. The cell body contains a cavity leading into the side of the cell body, which is arranged to be brought into contact with the surface of the metal sample body. In the cavity of the cell body, a Ni-NiO counter electrode is arranged together with a further Ni-NiO electrode which serves to control 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 cellulose sponge which is placed in the cavity of the cell body. The cellulose sponge constitutes a carrier body for the electrolyte which is firstly in direct contact with the Ni-NiO counter electrode and secondly is brought into direct contact with the surface of the metal sample body when the cell is arranged in contact therewith. In this measuring cell, the reference electrode, the potentiostat and the counter electrode in the Devanathan cell described above are combined into a single Ni-NiO counter electrode. The measuring system further comprises an electronic measuring and timing control circuit including a current follower, an instrument panel and four time-set electronic units. The entire system is powered by batteries that have a six-hour operating capacity. The measuring system must be used for determining the hydrogen concentration

tion in steel, based on the electrochemical permeation

technique. As stated in the article, the current generated in the measuring cell contains transient contributions that originate in a passivation process immediately after the cell is assembled. While the permeation current itself is a function of t -1 ' 12, these transient contributions decrease with t<-1>. The sensitivity of the cell is approximately 1 pA/cm 2. While the first and second electronic units in the electronic measuring and timing control circuit serve the purpose of eliminating the above-mentioned transient passivation contributions, the third and fourth electronic units control a digital integrated

tor which performs a digital integration of the current which

is carried from the measuring cell within a predetermined time period which is determined by the mentioned third and fourth electronic devices

named. As indicated in the article, the integration of the current supplied from the measuring cell within a predetermined time period can be used to determine the initial concentration of hydrogen in the metal sample body. As will be understood from the above, the system is only adapted to provide a single determination of an initial hydrogen concentration in a metal sample body and is not adapted to provide continuous monitoring of a given measurement

place over a long period of time. The system must also be assembled before use and is therefore not a permanently closed, ready-to-use system. The system also requires a very carefully prepared electronic measuring and timing control circuit.

Brief description of the invention.

The method and device according to the present invention enables a stable and reliable measurement of hydrogen flow by means of units which can be produced at low costs and which do not require maintenance, and uses simple and reliable external measuring equipment.

The invention provides a device for determination

of hydrogen flow through a metal membrane, which is sel-

tively permeable to hydrogen, from a hydrogen input

side, comprising the metal membrane, an electrolyte solution

communicating electrolytically with the side of the membrane opposite the hydrogen input side, a metal/metal oxide cathode communicating electrolytically with the electrolyte,

wherein the cathode is capable of oxidizing hydrogen to hydrogen ions, and wherein the electrolyte and the cathode are contained in a housing in which the metal membrane forms a wall part, and leads for electrical connection with external current measuring equipment, which device is adapted to be held in a mainly short-circuited condition when directly connected between the wires or when connected to a current measuring system with low internal impedance. The measurement principle used according to the present invention is the Devanathan principle described above. According to the invention, however, the functions of the reference electrode, the potentiostat and the counter electrode are combined in a single component, i.e. the metal/metal oxide cathode.

The measuring device according to the invention thus only involves the use of two electrodes, one of which is made up of the membrane, which makes it possible to create a compact, simple and accurate measuring unit. To carry out the measurement, the only external equipment required is an ammeter. In order for the device according to the invention to function correctly, it is extremely important that the device is always kept in a short-circuited state, whether it is in use (short circuit through the ammeter) or not in use (short circuit through a direct connection between the wires).

A particular aspect of the present invention is

an inexpensive and compact device for determining hydrogen flow through a metal membrane, which is selectively permeable to hydrogen, from a hydrogen entry side, comprising the metal membrane, an electrolyte solution communicating electrolytically with the side of the membrane opposite the hydrogen entry side, and a metal/metal oxide cathode communicating electrolytically with the electrolyte, where the cathode is capable of oxidizing hydrogen to hydrogen ions, and where the electrolyte and the cathode are contained in a housing in which the membrane forms a wall part, which housing is permanently sealed. In this aspect of the present invention, a measuring device can be provided which has extremely small dimensions,

including small internal volume. Due to the permanent seal, a permanently high standard of purity and reproducibility can be achieved by completing the measuring device

is placed in a factory under controlled conditions, and no maintenance is required, in contrast to known hydrogen measuring devices.

The invention also relates to a method for determining hydrogen flow through a metal membrane, which is selectively permeable to hydrogen, from a hydrogen input side, comprising that:

one provides a housing, arranges a metal/metal oxide

cathode and an electrolyte solution in the housing, which cathode is capable of oxidizing hydrogen to hydrogen ions and communicates electrolytically with the electrolyte solution, arranges the metal membrane in the housing so that it forms a wall

part thereof, brings the side of the metal membrane opposite the hydrogen entry side into contact with the electrolyte solution, seals the metal membrane in the housing, short-circuits the metal membrane at the metal/metal oxide cathode, and measures the electric current generated as a result of the oxidation of passing hydrogen through the membrane.

Another aspect of the present invention concerns

a method for determining hydrogen flow through a metal membrane, which is selectively permeable to hydrogen,

from a hydrogen input side, comprising that:

one provides a housing, arranges a metal/metal oxide

cathode and an electrolyte solution in the housing, which cathode is capable of oxidizing hydrogen to hydrogen ions and communicates electrolytically with the electrolyte solution,

arranges the housing in contact with the metal membrane so that at least part of the metal membrane forms a wall part of the housing, brings the side of the metal membrane opposite the hydrogen entry side into contact with the electrolyte solution, seals said part of the metal membrane relative to the housing , short-circuits the metal membrane and the metal/metal oxide cathode and continuously measures the electric current generated as a result of the metal membrane and the metal/metal oxide cathode being short-circuited. In this aspect of the present invention, the escape of hydrogen from the side of the metal membrane

which is opposite the hydrogen input side, determined continuously by continuous measurement of the electric current. On the basis of the avoidance of hydrogen, the hydrogen flow through the metal membrane can be determined.

When carrying out the method according to the invention, the above-mentioned device according to the invention can advantageously

is reversed and gives highly reproducible and accurate measurement results.

Detailed description of the invention.

The metal membrane used according to the invention should be of a metal which is selectively permeable to hydrogen. Usable metals for this purpose are in practice steel/iron and palladium. For many purposes, the metal membrane is made of a sheet of steel/iron that is coated with palladium on one or both sides. The thickness of the membrane determines the in-direction measurement response time. A palladium membrane or

-coating is normally in the range 0.05-10 um, preferably 0.1-

0.5 ym, while a steel membrane can be considerably thicker,

typically from approx. 20 ym to approx. 1 mm.

The metal/metal oxide cathode used according to

to the present invention, should be of a type which is capable of oxidizing hydrogen to hydrogen ions, but which will not give rise to any undesirable oxidation reactions with other components present. Preferred cathodes used according to the invention thus have an oxidation potential which is between 200 and 1200 mV above the equilibrium potential of the hydrogen electrode at the pH of the electrolyte solution, in particular 600-100 mV above the equilibrium potential of the hydrogen electrode at the pH of the electrolyte solution. In the currently preferred embodiments, the cathode's oxidation potential is approx. 900 mV above the equilibrium potential of the hydrogen electrode at the pH of the electrolyte solution.

Subject to this, the metal oxide of the cathode can be chosen from, for example, silver oxide, mercury oxide and manganese dioxide. The metal in the metal/metal oxide cathode serves only as an electrical conductor, but should be of a nature that is essentially chemically inert in relation to the metal oxide and electrolyte solution. The metal can therefore suitably be chosen from nickel, nickel-plated steel, silver and precious metals such as

gold, palladium and platinum.

The potential of the metal/metal oxide cathode can also be expressed

with reference to the area required for anodic oxidation of hydrogen on the metal membrane, preferably i

the range -100 to +300 mV E„.

ride

The cathode should be designed so that it can supply current

to the metal membrane in sufficient quantity to oxidize all hydrogen passing through the membrane under normal conditions of use, while still remaining in the acceptable potential range.

Furthermore, the total capacity of the cathode should preferably exceed

increase the normal service life of the metal membrane.

The electrolyte solution that is used according to the invention and communicates with the metal membrane and the metal oxide at the cathode should be an electrolyte that is appropriately adapted to the nature of the metal oxide. Examples of electrolyte solutions include aqueous alkaline solutions such as sodium or potassium hydroxide, normally in concentrations in the range 0.1-14N. For some applications, for example when the measurement is to be carried out at high temperatures,

it may be preferred to use an acidic electrolyte

solution in combination with a suitable metal oxide, for example sulfuric 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. In this embodiment, the metal membrane can be a steel/iron membrane with a palladium coating on one or both sides, or it can consist of a palladium coating on the plastic foil. In this embodiment, the plastic foil can advantageously be arranged so that it forms an outer side of the device and provides mechanical protection for the metal membrane, and in order to provide access for hydrogen to the metal membrane, the plastic foil can be perforated or made of a hydrogen-permeable plastic material. By providing a perforator-

ing the supporting plastic film, any plastic material that is not affected by the relevant corrosive forces can be used, such as a film made of, for example, Mylar ® . Alternatively, the hydrogen-permeable plastic foil can be made of, for example, polytetrafluoroethylene (Teflon®).

In this embodiment of the device according to the invention, the metal/metal oxide cathode can also be supported on a plastic foil, preferably a hydrogen-impermeable plastic foil, which forms a wall part of the housing. The plastic foil that supports the metal/metal oxide cathode is preferably made of a strong, inelastic and flexible plastic material, such as Mylar®. When the metal membrane and the metal/metal oxide cathode supported on respective plastic foils are provided, the supporting foils can be kept at a distance from each other by means of a separator and can be connected and sealed to each other along an outer edge part. In this embodiment, a so-called "Patch Cell" is provided which is designed to be mounted on an outer surface of a steel or iron wall, which is to be examined with regard to corrosive effects. In this so-called "Patch Cell", the electrolyte solution can be a layer of an electrolyte paste which is arranged on the side of the metal membrane opposite the hydrogen entry side and consequently hidden inside the space defined between the metal membrane supporting foil and the metal /- metal oxide cathode supporting foil.

In a further embodiment of the invention, the device comprises a first device part and a second device part, both with any of the above-mentioned characteristics, in which the first device part constitutes an active sensing element and the second device part, which is sealed so that hydrogen is excluded, constitutes a reference element. In this embodiment, the second device part is adapted to react to any environmental influences other than the corrosive forces or influences which are being measured by means of the first device part. The second device part can thus constitute a temperature-responsive sensor. When connected with suitable measuring equipment, environmental influences other than the corrosive influences which are to be determined using the measurement results provided by the first device part can be compensated for using measurement results provided by the second device part. Such environmental influences other than corrosive influences can, for example, be temperature variations.

In an important embodiment of the method according to the invention, the hydrogen flow is generated as a result of a chemical or electrochemical reaction between the membrane's input surface and an external fluid medium such as a liquid or gas with which the membrane's surface 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 a chemical or electrochemical reaction, so that the hydrogen flow measured by the method according to the invention will be representative of the hydrogen flow of which the object is made the same material will be exposed to when exposed to the environment in question.

In one embodiment, the inlet side of the membrane is thus exposed to a fluid which corrodes the membrane's surface metal with the generation of a hydrogen flow into the membrane. An important example of this is when the membrane is made from a sample of a steel type, if hydrogen corrosion,

when subjected to chemical or electrochemical treatment-

is in a cleaning solution, in pickling, phosphating, stripping

or metal plating baths, or in other electrogalvanic processes, must be judged. The membrane can also be made of steel that is annihilated for corrosion in a process stream from 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, whose protective effect is to be studied under corrosive or other influences. Alterna-

tively, the metal membrane can be provided with an outer coating or layer of a material whose corrosive effect on the metal membrane is to be studied.

The method according to the invention can also be used

for determining the hydrogen absorption in steel exposed to corrosion in natural environments, which can be from atmospheric exposure to natural water and soil, including exposure to or in building materials, such as concrete, heat-insulating materials, wood etc.

According to the invention, the metal membrane can expo-

for said surroundings or materials, also including the case where solid materials are applied to mem-

the fire, and the case where the device is built into or applied to a material whose hydrogen influence on the steel is to be monitored.

The membrane can be supplied with surface coatings corresponding to the desired simulation, including chemically corrosive layers, paints, metal coatings, anti-corrosive protective coatings, including paints etc., whose hydrogen uptake or whose effect on the membrane metal's hydrogen uptake is to be examined or monitored.

The size and shape of the exposed membrane can be adapted to the relevant purpose, for example so that the functions of a structural component are simulated.

The membrane may also be subjected to further electrochemical control or bimetallic contact to enable the effects of cathodic protection, galvanic action or any other means of control to be studied.

Furthermore, the method according to the invention can be used for control (testing and/or standardization) of test solutions in which stress corrosion or hydrogen embrittlement tests are carried out.

A very important example of the determination of hydrogen absorption in steel or iron that is exposed to corrosion in the natural environment is the determination of hydrogen absorption in a steel or iron wall in a pipe or container. In this case, the metal membrane is arranged near the outer surface of the steel or iron wall, and the flow of hydrogen escaping from an outer surface of the steel or iron wall is determined.

Brief description of the drawing.

The invention will now be described further with reference to the drawing, where

Figure 1 is a vertical cross-section of a first embodiment of a hydrogen probe or hydrogen monitoring cell according to the invention. Figure 2 is an illustration of a possible application of the hydrogen monitoring cell shown in Figure 1. Figure 3 is a vertical cross-section of a prototype of a measuring cell constructed in accordance with the principles of the

present invention.

Figure 4 is a diagram showing the current deposited against

the time in a test application of the prototype shown on

figure 3.

Figure 5 shows another embodiment of a hydrogen

probe or hydrogen monitoring cell according to the invention,

partly in average.

Figure 6 shows a third embodiment of a hycfrogen-

probe or hydrogen monitoring cell according to the invention,

partly in average.

Figure 7 shows a fourth embodiment of a hydrogen

probe or hydrogen-monitoring cell according to the invention or a "Patch Cell", partially in section.

Figure 8 is an exploded view of the embodiment or

"Patch Cell" which is shown in figure 7, and

Figure 9 shows a fifth embodiment of a hydrogen

probe or hydrogen monitoring cell according to the invention,

partly in average.

Detailed description of the drawing.

Figure 1 shows a first embodiment of a hydrogen monitoring cell according to the invention designated 10 as a whole. CEllen 10 is designed as a button-shaped energy cell

(a mercury battery) of the kind commonly used in

watches, calculators, hearing aids or the like. The cell 10 comprises two main components, a lower part and an upper part designated 11 and 12 respectively. The upper part 12 forms a membrane wall in the measuring cell and is made of thin steel plate comprising plate material of pure iron or steel of the type that is usually used for the manufacture of tinplate in the canning industry. The inner surface of the upper part 12, i.e.

the surface that faces the inside of the measuring cell is provided with a coating of palladium. The lower part 11 constitutes, together with a metal oxide filling 14, a metal/metal oxide cathode in the hydrogen monitoring cell. The metal/metal oxide cathode can be constructed in several ways using several combinations of metals and oxides, all of which are known from the construction of alkaline batteries. The lower part 11 is made of a metal that does not or only very slowly oxidizes

their in a solution at the potential of the cathode, such as nickel

or nickel-plated steel, stainless steel, or a noble metal such as silver, gold or palladium or a coating of these metals on steel or another suitable support. The metal oxide can be manganese dioxide, silver oxide, mercury oxide and other oxides either alone or in combination with metal powders, mercury or other additives that serve to

to provide 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 energy generating cells. The metal oxide filling 14 is press-fitted to the lower part 11 so that good contact is achieved between the metal and the metal oxide.

The two components, ie the lower part 11 and the upper

part 12, is sealed together, a sealing connection 15 being arranged within the inward-facing edge of the upper part 12. The thickness of the upper part 12 is mainly determined by the material's ability to form in the inward-facing edge. However, the thickness of the material also determines the measurement

the cell's service life and response time. It will be understood that a thicker thin sheet metal gives a longer service life, but reduces the response time for the monitoring cell. A thin plate thickness of 0.25 mm, which is commonly used in the canning industry, gives satisfactory results in terms of the above factors. The thickness of the palladium coating on the inner surface of the upper part 12 may be within the range of 0.05-10 ym, preferably 0.1-0.5 ym.

Inside 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 placed. The electrolyte 16 can be a pure solution of alkali hydroxide, preferably sodium or potassium hydroxide. in a concentration of 0.1-14 molar, preferably 1 molar. The separator 17 serves to prevent particles of metal oxide

in coming into contact with the palladium coating and can be made of polymer fibres, for example polypropylene fibres, which are usually used as separators in alkaline batteries.

In Figure 2, the first embodiment of the hydrogen probe or the hydrogen monitoring cell is shown mounted at the end of a pipe 18 and in electrically conductive connection therewith. The pipe 18 can be made of, for example, steel or iron and is provided with an inner flange part 19. The lower part 11 in the measuring cell 10 is insulated in relation to the pipe 18

by means of an insulating ring-shaped gasket 20 which is mounted in the space between the inner surface of the pipe 18 and the outer surface of the lower part 11. In a central bore 21 in the flange part 19, a spring contact 22 is mounted in an insulating gasket 23 .Through a soldered connection

24, the contact 22 is connected with an insulated metal wire 25

for establishing a connection with external current-measuring equipment which is also connected to the pipe 18 (not shown). A corrosion-resistant pipe 26 with an inward-facing flange 27 is mounted surrounding the pipe 18. The pipe 26 is insulated in relation to the measuring cell 10 by means of an insulating ring-shaped gasket 28 which is mounted between the flange 27 and the upper surface of the membrane 12 in the measuring cell. Furthermore, an insulating gasket (not shown) can be arranged in the space between the two pipes 18 and 26 to keep the pipes at a distance from each other. The measuring cell mounted at the end of the pipe 18 can thus be inserted or removed from a measuring environment in a manner known per se through one or more ball valves.

Figure 3 shows a prototype constructed according to the principles of the present invention. The prototype is designated 30 as a whole and comprises a housing consisting of a lower component 31 and an upper component 32 made of polymethacrylate. The two components 31 and 32 are connected to each other by means of screw connections 33 and 34 and enclose a membrane 35 in the space between the two components. The diaphragm 35 is made of a piece of thinned 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 lower component 31. The palladium coating is deposited from the following resolution:

per liter: 5 g PdCl2

240 g Na3P04, aq

55 g of (NH 4 ) 3 PO 4

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

Working conditions: Temperature 60-70°C,

2

current density 2 mA/cm ,

time 2-4 minutes.

A palladium coating with a thickness is thus obtained

of approx. 0.1-0.2 ym. The opposite, uncoated side of the steel membrane provides an exposed area of 7 cm 2 within

for a central circular recess in the upper component 32. The membrane 35 is attached and sealed to the inner

of the lower part 31 by means of an annular gasket 37 mounted in an annular recess 38 provided in the top surface of the lower component 31. In the interior of the lower component 31 two coaxially arranged cavities or chambers are provided respectively 39 and 40. The chamber 40 is filled with a metal/metal oxide electrode 41 which is connected by an insulated metal wire leading through a central bore 43 in the lower component 31. The metal/metal oxide electrode 41 is fixed in the chamber 40 by using a gasket 44 which also seals the chamber 39 in relation to the surroundings. Gaskets 37 and 44 are made of alkali-resistant rubber. The metal/metal oxide electrode consists of a nickel-plated steel shell which is filled with a mixture of equal amounts of mercury-silver oxide and a powder of metallic silver. The powders are pressed into the shell. The metal wire 42 is soldered to the outer surface or bottom surface of the nickel-plated steel shell. Inside the chamber 39 is contained an electrolyte solution of 1 N sodium hydroxide of high purity.

When mounting the cell 30, the cell is flushed with pure nitrogen and then placed in a degassed electrolyte solution. While the cell is immersed in the electrolyte solution, a vacuum is applied, which helps to remove gases before the cell is sealed.

Five cells of the type shown in Figure 3 have been constructed according to the principles described above. They were stored for 24 hours in a short-circuited condition. After the cells had been short-circuited for 24 hours, they generated currents in the range of 0.1-0.5 yA, typically 0.2 yA. As the internal area of the membrane" is 16 cm 2, the currents thus obtained correspond to a current density of approximately 0.01-0.03 yA/cm 2.

Figure 4 shows a diagram for current generated in a prototype cell of the type shown in Figure 3, plotted against time. The cell's zero current (not shown in figure 4) is approx.

0.2 yA as mentioned above. At time zero, the cell is immersed

in a 0.2 molar P^SO^ solution. During approx. 7 minutes, the current generated in the cell rises to a value

(125 yA) of approx. 50% of the maximum current (250 yA). After

to be stabilized at the maximum current, the cell is taken out of the solution at the time indicated by an arrow in Figure 4. The current decreases quite quickly. However, the transient rendering of the measuring cell can be increased (or decreased) by increasing (or decreasing) the thickness of the membrane. Renderings identical to that shown in Figure 4 were obtained in three successive measurements. The cell thus does not exhibit hysteresis.

In Figure 5, a second embodiment of the hydrogen measuring probe or hydrogen detection cell is shown, denoted by 50 as a whole. The hydrogen measuring probe 50 comprises a sensor element, i.e. a membrane, of a short length of a thin-walled steel or iron tube 51 which is closed at one end 51a. The inner surface of the tube 51 can be provided with

a palladium coating, although this is not necessary for the probe to function correctly. 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 type described above in connection with Figure 3, and which is pressed around a metal conductor 53 of, for example, nickel.

A separator 54 is arranged surrounding the metal/metal oxide electrode 52, 53. The separator can be made of, for example, propylene fibres. The assembled unit of the metal/metal oxide electrode 52, 53 and the separator 54 is fitted into a plastic plug 55 which is press-fitted into the open end of the tube 51 after the inner space in the tube has been filled with a degassed electrolyte solution 56, which can be of the type described above in connection with figure 3. The metal conductor 53 projects through the plug 55 and is connected to an insulated metal wire' 57. The pipe 54 is also connected to an insulated metal wire 58. The metal wires 57 and 58 are arranged to to

be connected to external current measuring equipment, for example a conventional sensitive ammeter. A plastic sheath 59 is heat-shrunk around the open end of the tube 51 and around the metal wires 57 and 58.

Figure 6 shows a third embodiment of the invention. The embodiment of the invention shown in Figure 6, denoted by 60 as a whole, deviates only slightly from the embodiment of the invention shown in Figure 5. The same reference numbers thus denote the same components in the two embodiments. In contrast to the probe 50, the probe 60 comprises a tube 61 of a metal which is impermeable to hydrogen, for example 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 which is opposite to the end equipped with the plastic plug 55 is provided with a steel or iron disc which is welded to the tube 61. The disc 62 forms the metal membrane of the hydrogen measuring probe according to the principles according to the present invention. The embodiments of the invention shown

on figure 5 and figure 6, is intended to be used and is designed to be used when measuring in situ, for example when the metal is subjected to an electro-galvanic process. Thus, the probe is dipped in the electro-galvanic solution to simulate the process normally carried out in the bath (for example cleaning, pickling, phosphating, stripping, electro-plating, electro-deposition etc.) while the tube in the hydrogen probe, i.e. respectively pipe 51 in the embodiment shown in Figure 5, and pipe 61 in the embodiment shown in Figure 6, are connected to external electro-galvanic current/voltage generating means.

The long and slender shape of the hydrogen probe 60 shown in Figure 6 provides additional useful features. The shape of the hydrogen probe thus makes it easy to adapt

to an "access device". Furthermore, the length of the hydrogen probe when combined with a tube of low thermal conductivity, for example a tube of stainless steel or iron/nickel alloy, makes it possible to apply cooling to the end of the tube which is provided with the plastic plug 55, while the end of the tube which is provided with the metal disc 62 is kept at a significantly higher temperature in the galvanic bath. The temperature dependence of the measurement is thus reduced. The combination of a hydrogen-impermeable metal tube 61 and hydrogen-permeable metal disk 62 makes it possible to carry out measurements on special, possibly user-supplied iron or steel materials.

Figures 7 and 8 show a fourth embodiment of the hydrogen measuring device or a so-called "Patch Cell". The "Patch Cell" shown in Figures 7 and 8 is designated 70 and is adapted to be mounted on an outer surface of a steel or iron wall which is subject to corrosion. Said "Patch Cell" is thus designed to detect and measure hydrogen that penetrates through the steel wall and is released from the outer surface of the iron or steel wall within a measuring range determined by the physical dimensions of the measuring device. Figure 7 is a vertical section of the measuring device 70 assembled and ready for use, while Figure 8 is an expanded image thereof. The same reference number thus denotes corresponding parts in Figures 7 and 8. The measuring device 70 comprises an outer protective foil 71 of a suitable plastic material which is flexible, strong and inelastic, for example 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 in the measuring device. The metal layer 72 can be made of any suitable metal, for example nickel or silver. Apart from the fact that the plastic foil 71 supports the metal layer 72, it provides protection and insulation for the metal layer in relation to the surroundings. The metal layer can be deposited as a thin metal coating on the plastic foil 71. The metal oxide component in the metal/metal oxide electrode is denoted by the reference number 73. The metal oxide can be made from a mixture of metal oxide powder and metal powder which is pressed into an intimate mixture , which also contains a binder if desired. The metal oxide component is applied to the metal component in the metal/metal oxide electrode, i.e. the metallic layer 72, for example by thick film technology.

Like the above described embodiments of the invention, the measuring device 70 comprises a metal membrane which consists of a foil 77 of a metal which is permeable to hydrogen, for example palladium or iron coated with palladium on both sides. Similar to the combination of the plastic foil 71 and the metal 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 for providing external access to the membrane 77. Alternatively, the plastic foil can be made of a material that is permeable to hydrogen, for example polytetrafluoroethylene (Teflon).

The metal oxide 73 in 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 can be made of a sheet of alkali-resistant filter paper, for example made of polypropylene fibres. The edge of the separator 74 is provided on both sides with layers 75 and 76 of a plastic material which penetrates the filter material and also functions as an adhesive and sealing compound when the measuring device is assembled in a heat sealing process which will be described in detail below. The material in the layers 75 and 76 must fulfill two functions, firstly the material must be able to penetrate and fill the pores in the separator, and secondly the material must have adhesive properties to be able to bind the metallic layer 72 and the metallic membrane 79 firmly together and seal the measuring device.

As will be understood, the plastic foil 71, the metallic layer 72 and the metal oxide component 73 are produced as a single unit to be connected and sealed with another unit comprising the metal membrane 77 and the plastic foil 78 by means of the adhesive edge material 75 and 76 of the separator 74 Before assembling the measuring device, an electrolyte solution is placed between the metal/metal oxide electrode and the metal membrane 77. The electrolyte solution is a degassed alkaline electrolyte which is preferably formed into a paste using techniques known from the manufacture of batteries. The metal oxide component 73 and the separator sheet 74 are saturated with electrolyte solution, and further a layer of the electrolyte solution is applied to the metal membrane 77. When assembling the cell, air (oxygen) must be excluded from the space between the metal/metal oxide electrode and the metal membrane. Furthermore, the electrolyte solution must be prevented from coming into contact with the adhesive material 75 and 76. The cell is assembled using a suitable heated tool comprising two parts which are brought into contact with the edge parts 71 and 78 respectively on the plastic foil and pressed together.

The plastic film 71, the metallic layer 72, the metal membrane

77 and the plastic film 78 are provided with tabs 71a respectively,

72a, 77a and 78a. The metal tab 72a is to be folded around a metal wire 82, and the metal tab 77a is designed to be folded around a metal wire 83, as shown in figure 7. The plastic tab 71a and the plastic tab 78a isolate the metal tabs in relation to each other and in relation to the surroundings. The metal threads 82

and 83 are intended to be short-circuited when the measuring device is stored and to be connected to external current-measuring equipment, such as a conventional ammeter, during use.

A cover made of paper or cardboard is placed on the outer surface of the plastic foil 78 using a compressible, flexible material 80 such as foam rubber or foam plastic with self-adhesive surfaces to protect the exposed surface areas of the metal membrane 79 when the measuring cell is not in use. The adhesive surface of the material 80 that faces the cover 81 also serves to attach the measuring device to a measuring point on a steel or iron wall.

In Figure 9, a fifth embodiment of the hydrogen measuring device according to the invention is shown and denoted by 90 as a whole. The embodiment of the invention shown in Figure 9 differs from the embodiments described above in that the device comprises two essentially identical parts or halves. The first part constitutes the active sensing element, and the second part is neutralized or inactive or arranged to be neutralized or inactivated so that the second part constitutes a reference element. Unlike the first part of the measuring device, the second part or the reference element is not intended to be exposed to corrosive forces or other influences to be measured. Instead, the reference element only reacts to temperature variations. Connected to suitably calibrated differential measuring equipment, the first part of the measuring device provides current measurements, while the second part or reference element serves temperature compensation purposes when current is supplied to the measuring equipment.

The measuring device 90 comprises a metal sheet 91, for example a nickel or silver sheet. The metal sheet 91 is common to both of the above-described parts of the measuring device and provides support for these and gives the measuring device as a whole physical strength. On a first side of the metal sheet on which the active sensing part of the measuring device is constructed, a metal oxide component 92 is placed 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. The first part of the measuring device further comprises an electrolyte solution which is enclosed in the space between the metal/metal oxide electrode and the metal membrane 94.

The above-described components of the measuring device are basically identical to the corresponding components in the embodiment according to the invention shown in Figures 7 and 8. A hydrogen-permeable plastic foil that serves protective purposes can thus also be arranged so that it covers the metal membrane 94 .

The second part of the measuring device comprises components which are basically identical to the components described above, i.e. a metal oxide component 95, a separator 96, a metal membrane 97 and an electrolyte solution enclosed in the space between the metal/metal oxide electrode and the metal membrane 97. The second part of the measuring device 90 further comprises a corrosion-resistant foil 98 which completely seals the second part of the measuring device and protects the metal membrane 97 when the measuring device is exposed to corrosive forces. The foil 98 can either form an integral part of the measuring device or be applied by a user. The foil 98 can thus be made of plastic or be applied as an anti-corrosion material, paste or the like.

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

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

It will be understood that the embodiments described above are only illustrative of the application of the principles of the present invention. Numerous other embodiments can be imagined by those skilled in the art without deviating from the idea and framework of the present invention.

Referrals

1. M.A.V. Devanathan and Z. Stachurski:

Proceedings of the Royal Society, 1962, Vol A270, p.90

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

3. GB Patent No. 1,585,070

4. GB Patent No. 1,524,017

5. G. Mansfeld, S. Jeanjaquet and D.K. roe,

Materials Performance, February 1982.

Claims (25)

1. Device for determining hydrogen flow through a metal membrane, which is selectively permeable to hydrogen, from a hydrogen entry side, comprising the metal membrane, an electrolyte solution communicating electrolytically with the side of the membrane opposite the hydrogen entry side, a metal/ metal oxide cathode communicating electrolytically with the electrolyte, which cathode is capable of oxidizing hydrogen to hydrogen ions, where the electrolyte and the cathode are contained in a housing in which the metal membrane forms a wall part, and wires for electrical connection to external current measuring equipment , which device is arranged to be kept in a substantially short-circuited state by direct connection between the wires or by connection to a current measurement system with low internal impedance. 2. Device for determining hydrogen flow through a metal membrane, which is selectively permeable to hydrogen, from a hydrogen entry side, comprising the metal membrane, an electrolyte solution communicating electrolytically with the side of the membrane opposite the hydrogen entry side, and a metal /metal oxide cathode communicating electrolytically with the electrolyte, which cathode is capable of oxidizing hydrogen to hydrogen ions, where the electrolyte and the cathode are contained in a housing in which the membrane forms a wall part, and where the housing is permanently sealed. 3. Device according to claim 1 or 2, where the membrane is a palladium membrane. 4. Device according to claim 1 or 2, where the membrane comprises iron or steel with a palladium coating on one or both sides, which has a thickness in the range 0.1-10 ym, preferably 0.1-5 ym. 5. Device according to any one of claims 1-4, where 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 electrolyte solution. 6. Device according to claim 5, where the oxidation potential of the cathode is 600-1200 mV above the equilibrium potential of the hydrogen electrode at the pH of the electrolyte solution. 7. Device according to claim 5, where the oxidation potential of the cathode is approx. 900 mV above the equilibrium potential of the hydrogen electrode at the pH of the electrolyte solution. 8. Device according to any one of claims 1.-7, where the metal oxide of the cathode is selected from silver oxide, mercury oxide and manganese dioxide, and the metal in the cathode is selected from nickel, nickel-plated steel, silver and precious metals such as gold, palladium and platinum. 9. Device according to claim 8, where the electrolyte is a degassed alkaline solution such as sodium or potassium hydroxide with a concentration of 0.1-1 OM. 10. Device according to any one of the preceding claims, where the metal membrane is supported on a plastic foil. 11. Device according to claim 10, where the plastic foil forms an outer side of the device and is perforated to provide hydrogen access to the metal membrane. 12. Device according to claim 10, where the plastic foil forms an outer side of the device and is made of a hydrogen-permeable plastic material. 13. Device according to any one of the preceding claims, where the metal/metal oxide cathode is supported on a hydrogen-impermeable plastic foil which forms a wall part of the housing. 14. Device according to any one of claims 10-12 and 13, wherein the metal membrane supporting foil and the metal/metal oxide cathode supporting foil are kept at a distance from each other by means of a separator and joined and sealed to each other along an outer edge part. 15. Device according to claim 14, where the electrolyte solution is a layer of an electrolyte paste which is arranged on the side of the metal membrane which is opposite the hydrogen input side. 16. Device according to any one of the preceding claims, comprising a first device part and a second device part which both have any of the above-mentioned characteristics, where the first device part constitutes a active sensing element, and the second device part, which is sealed so as to exclude hydrogen, constitutes a reference element. 17. Method for determining hydrogen flow through a metal membrane, which is selectively permeable to hydrogen, from a hydrogen input side, comprising: that one provides a housing, arranges a metal/metal oxide cathode and an electrolyte solution in the housing, which cathode is capable of oxidizing hydrogen to hydrogen ions and communicates electrolytically with the electrolyte solution, arranges the metal membrane in the housing so that it forms a wall part thereof, brings the side of the metal membrane opposite the hydrogen entry side into contact with the electrolyte solution, seals the metal membrane in the housing, short-circuits the metal membrane and the metal/metal oxide cathode, and measures the electrical current generated as a result of oxidation of hydrogen passing through the metal membrane. 18. Method for determining hydrogen flow through a metal membrane, which is selectively permeable to hydrogen, from a hydrogen input side, comprising: that one provides a housing, arranges a metal/metal oxide cathode and an electrolyte solution in the housing, which cathode is capable of oxidizing hydrogen to hydrogen ions and communicates electrolytically with the electrolyte solution, arranges the housing in contact with the metal membrane so that at least part of the metal membrane forms the wall part of the housing, brings the side of the metal membrane opposite the hydrogen entry side into contact with the electrolyte solution, seals said part of the metal membrane in relation to the housing, short-circuits the metal membrane and the metal/metal oxide cathode and continuously measures the electric current generated as a result of the metal membrane and the metal/metal oxide cathode being short-circuited. 19. Method according to claim 17 or 18, where the hydrogen flow is generated as a result of a chemical or electrochemical reaction between the entrance surface of the membrane and an external fluid medium such as a liquid or gas with which the surface of the membrane reacts chemically or electrochemically. 20. Method according to claim 19, where the entrance side of the membrane is exposed to a fluid which corrodes the membrane's surface metal while generating a hydrogen flow into the membrane. 21. Method according to claim 19 or 20, where the membrane is made from a sample of a steel type, if hydrogen corrosion, by exposure to chemical or electrochemical treatments in a cleaning solution, in pickling, phosphating, stripping or metal plating bath, or in other electrogalvanic processes, must be assessed. 22. Method according to claim 19 or 20, where the membrane is made of steel which is exposed to corrosion in a process flow from a chemical plant or a power plant or when measuring or processing natural gas, oil or geothermal water vapour. 23. Method according to any one of claims 19-22, where the metal membrane is provided with an outer protective coating or layer, the protective effect of which is to be studied when exposed to corrosive or other influences. 24. Method according to any one of claims 19-22, where the metal membrane is provided with an outer coating or layer of a material whose corrosive effect on the metal membrane is to be studied. 25. Method according to claim 17 or 18, where the flow of hydrogen escaping from an outer surface of a steel or iron wall in a pipe or a container is determined, and where the metal membrane is arranged adjacent to the outer surface of said steel or iron wall .