GB2199951A

GB2199951A - Testing steel for hydrogen embrittlement

Info

Publication number: GB2199951A
Application number: GB08700995A
Authority: GB
Inventors: Morteza Shirkhanzadeh
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-01-17
Filing date: 1987-01-17
Publication date: 1988-07-20
Anticipated expiration: 2007-01-17
Also published as: GB2199951B; GB8700995D0

Abstract

An indication of embrittlement of a steel specimen due to hydrogen permeation during exposure to a test solution under simulated hydrodynamic conditions is provided by immersing into a test solution a rotating cylindrical cell comprising an electrolyte and a plurality of electrodes one of which is a membrane electrode made of a steel to be tested, applying a voltage between the electrodes sufficient to oxidize hydrogen diffusing out of the membrane electrode and measuring the cell current. As shown, the cell comprises a cylindrical membrane electrode 36, a pair of tubular separators 32 and 34 for retaining an electrolyte made of 0.2N NaOH, a Ni/NiO reference electrode 24 in the form of a cylindrical screen and a counter electrode 20 which is made of a platinum plated copper tube. An electrical motor (not shown) drives the cell at a constant rotation rate such that a turbulent flow is simulated within the test solution which surrounds the electrode in a cylindrical container. The cell is connected to a potentiostat. <IMAGE>

Description

Method and apparatus for indicating hydrogen embrittlement in steel due to corrosion under simulated field hydrodynamic conditions.

The present invention relates to a method and apparatus-for simulating turbulent hydrodynamic conditions and providing an indication of hydrogen embrittlement in steel due to corrosion under simulated conditions. The invention is particularlyuseful,and is therefore described below, for simulating turbulent hydrodynamic conditions which may prevail in process plants and providing an indication of hydrogen embrittlement in steel due to corrosion under such simulated conditions.

The behaviour of hydrogen in metals, particularly steel, has received considerable attention over the years because serious embrittlement and subsequent failure occur at relatively high yield strengths. The source of Failurs s failures iftencorrosion reaction at the steel surface. At the corrosion potential, the dissolution of iron, Fe > Fe++ + 2e (anodic reaction), is accompanied by the cathodic reaction, 2H+ + 2e + 2H (ads). The hydrogen atoms which are produced at the steel surface may recombine to form H2 or may be absorbed by the steel as atomic hydrogen, subsequently causing embrittlement in metal.

Since the important role of hydrogen in the catastrophic failure of metals has been recognized, it is understandable that a great interest exists to explore ways of keeping the hydrogen out of the steel through manipulation of surface corrosion kinetics or modification of the steel's internal structure. In this respect, many attempts have been made to retard hydrogen entry into metal by selecting suitable alloys and/or by the use of corrosion inhibitors. In corrosion laboratories, the hydrogen permeation rate is usually measured to provide an indication of hydrogen embrittlement in metals. This method may also be employed to provide a basis for material selection and to substantiate the effect of inhibitors in retarding hydrogen entry into metals. Various measurement techniques have been proposed for the measurement of hydrogen permeation into metals.In certain techniques, the hydrogen atoms diffusing out of a sample are captured by a vacuum extraction technique and quantified by a volumetric or manometric procedure. These methods can be used for the determination of both the hydrogen content in the metal and the permeation rate of hydrogen through a membrane. The accuracy of these methods may prove unsatisfactory, however, whenever fairly low hydrogen concentrations are involved, or for low mobility of hydrogen in the metal. The volumetric and manometric methods are laborious and difficult to record automatically.

As to the measurement of the permeation rate of hydrogen through metal membranes, the above mentioned drawbacks have been eliminated with the development of a highly sensitive and convenient electrochemical method. In this technique, a thin section of the test specimen is fitted between two electrochemical cells. The specimen acts a5 a -common electrode to both sides.

Hydrogen, which can form at the input side during exposure to a corrosion test solution, will diffuse through the specimen and appear on the metal surface in the other cell. The output side of the specimen is potentiostated anodically using reference and counter electrodes in this cell, containing 0.2NNaOH.

so that any hydrogen which appears is immediately oxidized. The oxidizing current is therefore a measure of the hydrogen permeation. In this technique, hydrogen permeation measurement is carried out under stagnant conditions, i.e. there is no relative motion between the specimen and the corrosion test solution. However, many actual cases of embrittlement induced failures in industry occur in turbulent flow systems where there is vigorous relative motion between the corroding metal and its environment. In this respect, the electrochemical technique available for the measurement of the hydrogen permeation rate does not often represent the field hydrodynamic conditions. The relative motion between metal and its corrosive environment can effect processes occurring under static conditions.In particular, turbulent flow can have a marked effect on corrosion processes and on the rate of hydrogen entry into metals by influencing both the surface concentration of the active species and the film formation process on the metal surface. It would be most desirable therefore to employ a simple and convenient technique, which can simulate turbulent hydrodynamic conditions within a confined compartment, and which permits the hydrogen permeation rate to be measured electrochemically under such simulated conditions.

According to one aspect ofthe present invention, there is provided a method for simulating turbulent hydrodynamic conditions within a confines test solution and indicating embrittlement, due to hydrogen permeation, of a steel specimen during exposure thereof to said test solution under simulated turbulent hydrodynamic conditions, comprising the steps of immersing a cylindrical cell in a test solution contained in a concentric cylindrical tank, followed by rotating the said cell about its axis at a constant angular velocity so that a turbulent flow is simulated within the said tank, said cell having an electrolyte and a plurality of electrodes including a thin-walled cylindrical membrane electrode which is made of steel to be tested, and the outer face of which is exposed externally of the cell for contact with said test solution; applying a voltage between said plurality of electrodes sufficient to oxidize the hydrogen diffusing out of the-membrane electrode; and measuring the change in current beween said plurality of electrodes thereby providing a measurement of the hydrogen permeation from the test solution into said membrane electrode while it is externally exposed to the test solution under simulated hydrodynamic conditions.

According to another aspect of the invention, there is provided apparatus for simulating turbulent hydrodynamic conditions within a confined test solution and indicating embrittlement, due to hydrogen permeation, of a steel specimen during exposure thereof to said test solution under simulated turbulent hydrodynamic conditions, comprising: a cylindrical cell for immersion in the test solution during the test, said cell having an electrolyte retained within a pair of porous separators and a plurality of electrodes including a thin-walled cylindrical membrane electrode, which is made of a steel to be tested, and the outer surface of which is exposed externally of the cell for contact with the test solution, and the inner face of which is in contact with said electrolyte inside the cell; a tank to hold the test solution and to form a stationary concentric cylinder around the said cell; a holder assembly to which said plurality of electrodes are secured; a driving unit including a driving shaft for rotating the said cell at a constant angular velocity while it is immersed in the test solution, thereby providing simulated turbulent hydrodynamic conditions within the test solution; a slip-rings assembly and matching brush set to pick up the electrical signals from said plurality of electrodes; means for applying a voltage between said plurality of electrodes sufficient to oxidize the hydrogen diffusing out of the membrane electrode while it is immersed and rotating inside the test solution; and means for measuring the change in current between said plurality of electrodes thereby providing a measurement of hydrogen permeation from the test solution into the said membrane electrode as it is exposed externally to the test solution under simulated turbulent hydrodynamic conditions.

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a transverse sectional view of the lower portion of the driving shaft, the holder assembly and the cell of this invention before the membrane electrode is fitted to the cell; Figure 2 is a transverse sectional view of the lower portion of the driving shaft, the holder assembly and the cell of this invention after the membrane electrode is fitted to the cell; and, Figure 3 is a general illustration of this invention as used in a hydrogen permeation test.

Referring to Figure 1, the hoTder assembly of this invention comprising a stainless steel tubular outer casing 2 having the integral enlarged section 4 at its lower portion which is provided with a set of the grob screws 6 (only two are shown), equally positioned around its perimeter. Casing 2 is also provided with an electrical conductor 8 at its upperportion.The holder assembly of this invention further comprises an insulating body 10 which may be made of plastics (such as polytetrafluroethylen) and is press fitted inside the casing 2. Body 10 is provided at its upper portion with a stainless steel shroud 12 having a collar 14 and set of grob screws (not shown) for securing the holder assembly onto the driving shaft 16.Body 10 is also provided at its lower end with a silicon rubber ring 18 having an outer diameter slightly larger than the outer diameter of the body 10.

Referring to the same drawings in Figure 1, the cell of this invention comprises a counter electrode 20 made of a platinum plated copper tube which is internally covered by a thin coat 22 of an insulating resin,and a Ni/NiO reference electrode 24 in the form of a thin walled cylinder, the lower half of which is porous, forming a cylindrical screen. Counter electrode 20 is press fitted within the central hole of an annular spacer 26 which is made of plastics (such as polytetrafluroethylen), while the reference electrode 24 is secured suitably onto the annular spacer 26 such that it forms a concentric cylinder around the counter electrode 20. The fabricated assembly composed of reference electrode 24, spacer 26 and the counter electrode 20 is partly inserted into the lower portion of the body 10 and secured thereto by means of a suitable adhesive bonding.The electrical connections are made to the counter electrode 20 and reference electrode 24 by means of electrical conductors 28 and 30 respectively. The cell in this invention also comprises a pair of tubular separators 32 and 34 made of a flexible and porous material for retaining an electrolyte solution which is made of 0.2N NaOH. One separator (32) is press fitted into the annular space between the counter electrode 20 and the reference electrode 24 while the other separator (34) is press fitted onto the lower half of the reference electrode 24 such that the reference electrode 24 is sandwiched between the two separators 32 and 34.

The cell of this invention further comprises a membrane electrode designated A which includes a thin-walled cylindrical tube 36 made of a steel to be tested and adapted as a test specimen. Tube 36 has an internal diameter slightly larger than the outer diameter of body 10 and is closed at its lower end by a stopper 38 made of plastics (such as polytetrafluroethylen) which is suitably fitted into it to provide a satisfactory seal. The external surface of the membrane electrode A is partially coated with a thin coat 40 of a resin (such as methyl methacrylate) so that only two circular bands, one close to the upper end,and the other away from the two ends of tube 36, are left uncoated.

The membrane electrode A is assembled to the holder assembly of the invention as follows. The lower portion of the body 10 (holding counter electrode 20, reference electrode 24 and separators 32 and 34) is firstly dipped into a solution of 0.2N NaOH and then inserted coaxially into the tube 36 such that the counter electrode 20, reference electrode 24 and the separators 32 and 34 are all rested on the stopper 38 as shown in Figure 2. In this process, separator 34 is interposed between the reference electrode 24 and the inner surface of the tube 36. On the other hand, the rubber ring 18 seals the cell from the top allowing only a fixed area of the inner surface of the tube 36 to come into contact with the said electrolyte.At the end of this process, the membrane electrode A is secured to the holder assembly by a set of grob screws 6 (only two are shown) which also provide electrical contact between the membrane electrode A and the outer casing 2. Casing 2 is thus adapted as an electrical means to connect membrane electrode A to conductor 8.

The driving shaft 16 of this invention shown in Figures 1 and 2 is provided with two longitudinal grooves 42 cut on opposite sides of it to facilitate the passage of the electrical conductors 8, 28 and 30, upwards through the bearing unit 44.

The Ni/NiO reference electrode 24 in this invention may be formed by conventional anodic treatment of nickel Tn alkaline solutions to form nickel hydroxide followed by heating to convert the hydroxide to nickel oxide.

Separators 32 and 34 for use in this invention have to be physically flexible, chemically inert and insoluble in the NaOH electrolyte and preferably have a porosity of about 25 per cent or more, so as to permit the sodium hydroxide electrolyte to permeate through and contact the counter electrode, reference electrode and the inner surface of the membrane electrode. Suitable separators for use in this invention are the non-woven glass separators, preferably those separators that incorporate long glass fibres along with short glass fibres since such a combination increases the tear strength of the separators, thereby making themeasier to handle.

In Figure 3, a general illustration of this invention is shown as used in a hydrogen permeation test. According to this figure-, the cell of this invention, generally designated 50, is immersed in a test solution contained in tank 52. Tank 52 is secured to a supporting frame 54 and positioned such that it constitutes a concentric cylinder around the cell 50. Cell 50 is connected to the driving shaft 16 via the holder assembly of this invention, which is generally designated 56. Shaft 16 includes a bearing unit 44 secured to a supporting frame 58. Shaft 16 also carries a slip-rings assembly 60 which is provided with a matching brush set 62. Conductors 8, 28 and 30 (shown in Figures 1 and 2) are electrically connected to conductors 64, 66 and 68 respectively by means of the slip-rings assembly 60 and brush set 62.

At its upper end, shaft 16 is connected to a variable speed electrical motor 70 which is secured to a supporting frame 72. Motor 70 is provided with a tachogenerator unit 74 and a speed controller 76, thereby driving cell 50 at a constant rotation rate while cell 50 is immersed in the test solution. It is thus possible to simulate turbulent hydrodynamic conditions within the test solution. The application of a rotating cylinder for the simulation of turbulent hydrodynamic conditions is a well-known and well-studied technique; for example see a review by D.R. Gabe in The Journal of Applied Electrochemistry 1974, Volume 4, page 91, and the references therein.

Referring to Figure 3, a potentiostat 78 provides the power supply for the electrodes within the cell 50, and includes a connection WE for conductor 64 to the working electrode (membrane electrode), a connection CE for conductor 66 to the counter electrode 20, and a connection RE for conductor 68 to the reference electrode 24. The hydrogen atoms discharged from the test solution, due to the corrosion reactions on the surface of membrane electrode A, diffuse into the membrane and emerge on the inner surface of the membrane electrode.

Potentiostat 78 maintains on the inner surface of the membrane electrode a potential sufficient to oxidize the hydrogen diffusing out of the membrane.

These hydrogen atoms are instantly ionized quantitatively under the applied potential. As a result, there is an increase in current flowing between the membrane electrode and the counter electrode. This increase in current is measured by a recorder 80 which provides a measurement of the quantity of hydrogen diffusing through the membrane electrode.

It is thus seen that the invention permits simulation of turbulent hydrodynamic conditions within a test solution, and provides measurement of hydrogen permeation under the simulated conditions using a membrane which may be constructed of a type of steel to be tested. The design of the membrane electrode in this invention is such that it can be easily assembled to, or de-assembled from,the holder assembly. Thus, steels with various alloying elements can be tested for susceptibility to hydrogen embrittlement under simulated hydrodynamic conditions. The outer face of the membrane electrode can be further polarized either cathodically or anodically using conventional electrochemical techniques,and hence the effect of such polarization on the hydrogen embrittlement of metals can be examined.

The test solution for use in this invention may be, for example, an artificial corrosion test solution or a sample solution from an actual process plant.

The test solution may also contain corrosion inhibitors. In this case, measurement of hydrogen permeation can be used to substantiate the effect of inhibitors in retarding hydrogen entry into metals and to provide a basis for screening the inhibitors for such applications under turbulent flow conditions.

Claims

Claims

1A method of simulating turbulent hydrodynamic conditions within a confined test solution and indicating embrittlement, due to hydrogen permeation, of a steel specimen during exposure thereof to said test solution under simulated turbulent hydrodynamic conditions, comprising the steps of immersing a cylindrical cell in a test solution contained in a concentric cylindrical tank, followed by rotating the said cell about its axis at a constant angular velocity so that a turbulent flow is simulated within the said tank, said cell having an electrolyte and a plurality of electrodes includng a thin-walled cylindrical membrane electrode which is made of steel to be tested, and the outer face of which is exposed externally of the cell for contact with said test solution; applying a voltage between said plurality of electrodes sufficient to oxidise the hydrogen diffusing out of the membrane electrode; and measuring the change in current between said plurality of electrodes thereby providing a measurement of the hydrogen permeation from the test solution into said membrane electrode while it is externally exposed to the test solution under simulated hydrodynamic conditions.
2 The method according to claim 1, wherein measurement of the hydrogen permeation is used to test various steels for susceptibility to hydrogen embrittlement and to provide a basis for their selection.
3 The method according to claim 1, wherein said test solution contains corrosion inhibitors.
4 The method according to claim 3, wherein measurement of the hydrogen per meation is used to substantiate the effect of inhibitors in retarding hydrogen entry into metals and to provide a basis for screening the inhibitors for such application.
5 The method according to claim 1, 2, 3 or 4 wherein said test solution is an artificial corrosion test solution.
6 The method according to claim 1, 2, 3 or 4 wherein said test solution ts a sample solution from an actual process plant.
7 The method according to either of claims 5 or 6 wherein said membrane electrode is externally polarized.
8 Apparatus for simulating turbulent hydrodynamic conditions within a confined test solution and indicating embrittlement, due to hydrogen permeation, of-a steel specimen during exposure thereof to said test solution under simulated turbulent hydrodynamic conditions, comprising: a cylindrical cell for immersion in the test solution during the test, said cell having an electrolyte retained within a pair of porous separators and a plurality of electrodes including a thin-walled cylindrical membrane electrode, which is made of a steel to be tested, and the outer surface of which is exposed externally of the cell for contact with the test solution, and the inner face of which is in contact with said electrolyte inside the cell; a tank to hold the test solution and to form a stationary concentric cylinder around the said cell; a holder assembly to which said plurality of electrodes are secured; a driving unit including a driving shaft for rotating the said cell at a constant angular velocity while it is immersed in the test solution, thereby providing simulated turbulent hydrodynamic conditions within the test solution; a slip-rings assembly and matching brush set to pick up the electrical signals from said plurality of electrodes; means for applying a voltage between said plurality of electrodes sufficient to oxidize the hydrogen diffusing out of the membrane electrode while it is immersed and rotating inside the test solution; and means for measuring the change in current between said plurality of electrodes,thereby providing a measurement of hydrogen permeation from the test solution into the said membrane electrode as it is exposed externally to the test solution under simulated turbulent hydrodynamic conditions.
9 Apparatus according to claim 8, wherein said cell also includes R caltnter electrode, a reference electrode and means for removably securing said membrane electrode.
10 Apparatus according to claim 9 further including a potentiostat for applying the electrical voltage to the electrodes and a recorder for measuring said change in current.
i l A method of simulating turbulent hydrodynamic conditions within a confined test solution and indicating embrittlement, due to hydrogen permeation, of a steel specimen during exposure therebf to said test solution under simulated turbulent hydrodynamic conditions substantially as described with reference to and as illustrated in the accompanying drawings.
12 Apparatus for simulating turbulent hydrodynamic conditions within a confined test solution and indicating embrittlement, due to hydrogen permeation, of a steel specimen during exposure thereof to said test solution under simulated turbulent hydrodynamic conditions substantially as described with reference to and as illustrated in the accompanying drawings.