ASSESSING STEELS FOR UNDESIRABLE PHASES

Abstract: A method for assessing the presence of undesirable phases in a steel sample. An anodising current is applied to the sample such that the sample is exposed to an anodising potential. The rate of increase in the anodising potential is monitored, a relatively low rate of increase in the anodising potential indicating the presence of undesirable phases. The sample may be passivated before the anodising current is applied.

Description: ASSESSING STEELS FOR UNDESIRABLE PHASES The present invention relates to a method for assessing the presence of undesirable phases in steel samples Certain types of high-alloy stainless steel are subject to the formation of deleteπous particles in their micro structure duπng heat treatment and subsequent cooling, especially of large forgmgs This problem aπses in particular with duplex stainless steels, which consist nominally of equal volume fractions of two metallic phases (ferπte and austemte) The problem is more likely to occur m so-called superduplex steels, which have high chromium, molybdenum, nickel and nitrogen contents Particles of bπttle chromium πch phases, most commonly sigma (σ) phase, form in the temperature range of roughly 1000°C to 600°C These particles can destroy the normal ductile properties of the steel, rendeπng it subject to bπttle fracture and hydrogen embnttlement (and also enhanced maπne coπosion) Other undesirable phases similar to sigma include cm (χ) phase and R phase These phases are higher in molybdenum than sigma A relatively harmless phase that can also form at these temperatures is chromium nitπde It has been recognised for some years that sigma phase can be detected electrochemically using a technique generally referred to as potentiodynamic reactivation or potentiokmetic reactivation and known as the EPR test This is a test first proposed in the 1970s which has been used extensively to monitor welded stainless steels for a phenomenon called sensitization This phenomenon is due to the precipitation of an undesirable phase (chromium carbide), which reduces the corrosion resistance of a region near each weld The EPR test procedure has been used to detect sigma phase in duplex stainless steels However, the EPR test does not detect harmful phases themselves but rather detects depletion of chromium and/or molybdenum m the base steel around each particle of the phase in question These elements are beneficial for corrosion resistance, so the depleted zones have less corrosion resistance In the EPR test a controlled electrode potential is first applied to the steel m a three-electrode electrochemical cell, to form a uniform passivating oxide film on the surface of the stainless steel, with an associated low electrochemical (ionic) current flowing m the solution The solution is always an acid, and generally this is sulphuπc acid containing a sulphur compound such as potassium thiocyanate or thioacetamide to make the environment more aggressive The applied potential is then reduced until at a certain value the oxide film is destroyed and the metal passes into the actively corroding state with a higher current By adjusting the solution and the rate of vaπation of the potential, the test can be configured so that only the depleted zones suffer this reactivation process By measuπng the amount of electπcal charge passed duπng reactivation, a correlation with the amount of chromium depletion and hence with the amount of harmful phase may be deduced EPR tests have been used m the field as well as the laboratory, but suffer from a number of recognised disadvantages such as complex control hardware, use of aggressive acids, need for de-oxygenation of the acid, different procedures and calibrations required for every steel, limited sensitivity, and general slowness There is however a further overriding disadvantage that makes the EPR procedure less appropπate for the detection of sigma phase, that is although the sigma phase itself has roughly the same composition whether formed at low or high temperature, the associated depleted zone has a radically different composition depending on temperature The higher the temperature, the less the amount of chromium and molybdenum depletion and the more danger that the EPR test will give a false negative result It is an object of the present invention to provide an improved electrochemical test capable of detecting the presence of sigma or similar phases According to the present invention, there is provided a method for assessing the presence of undesirable phases m a steel sample, wherein an anodising current is applied to the sample such that the sample is exposed to an anodising potential, and increases in the anodising potential are monitored, a relatively low rate of increase m the anodising potential indicating the presence of undesirable phases The sample may be passivated by immersion in an alkali solution Alternatively or in addition the sample may be passivated by applying an anodising current to the sample The rate of increase in the anodising potential may be monitored for example by momtoπng the slope of a curve representing the vaπation of anodising potential with time, or by measuπng the anodising potential a predetermined time after the initiation of the anodising cuπent, or by measuring the time taken for the anodising potential to reach a predetermined potential. The invention provides an electrochemical procedure and associated electrochemical probe that make it possible to measure the amount of chromium-rich phases in stainless steel either directly or by detecting chromium depleted regions. It has been developed for 25% chromium duplex stainless steels but would work on other materials including less-alloyed duplex steels, weld metals, and high-alloy austenitic steels. Indeed the test has features that make it uniquely flexible when used on different steels. An example of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates the variation with time of an anodising potential applied in accordance with the method of the present invention; Figures 2 to 6 illustrate respectively the linear fit of each of five curves shown in Figure 1 ; Figure 7 illustrates the reproducibility of data such as that shown in Figure 1 ; Figures 8 and 9 show results indicating the potential impact of thermal pre- treatment to variations in the monitored potential; Figure 10 is a series of micrographs which illustrate the micro structures of specimens tested using the invention; Figure 11 illustrates Charpy impact failure as a function of Ferrite meter reading for the specimens; Figures 12 and 13 illustrate the variation over time of the potential at the specimens when the method is applied with different cuπents; Figure 14 illustrates the potential at the specimens after a particular time for different currents; Figure 15 illustrates the time taken for the potential at the specimens to reach a particular value for different currents; Figures 16, 17 and 18 illustrate variations with time of anodising potentials applied to a sample in a 5% NaCl solution; Figure 19 shows results similar to those of Figure 1 but with a modified pre- passivation treatment; Figure 20 illustrates vaπations with time of potentials applied to a seπes of specimens in a 5% NaCl solution, Figure 21 illustrates vanations with time of potentials applied to the seπes of specimens in the 5% NaCl solution at 50C, Figure 22 illustrates vaπations of time of potentials applied to the seπes of specimens in a more concentrated NaCl solution, Figure 23 illustrates the potential at the senes of specimens after 500s for a range of temperatures Figure 24 illustrates vaπations with time of potentials applied to a seπes of specimens in a bromide solution, and Figure 25 illustrates the Charpy failure energy as a function of the potential at the specimens after 500s The data represented m Figures 1 to 7 was generated using a prefeπed double passivation procedure in accordance with the present invention That procedure used a mildly alkaline solution for initial passivation, although a neutral pH may also work A sodium sulphate/sodium hydroxide solution of pH 12 and a preferred sodium carbonate solution of pH about 11 5 have been used with very similar results In alkaline solutions none of the main alloying elements m stainless steel are soluble, and the steel behaves in a completely passive manner, until the electrode potential reaches the positive value such that chromium and molybdenum become oxidised to soluble forms (chromate and molybdate respectively) An example of one double passivation procedure in accordance with the present invention is set out in paragraphs 1 to 7 below 1 Wet-abrade (preferably polish) the steel surface and dry 2 Immerse in a mildly alkaline environment (no de-aeration necessary) 3 Pre-passivate (anodise) the steel by applying a constant potential or preferably a constant, relatively large cuπent density (e g lOOμA/cm"), using a three- electrode electrochemical cell In the case of constant cuπent density, the potential would increase (become more positive) with time The constant potential, or the final potential to be reached at the constant cuπent density, are chosen to be just below the range of potential where the sigma phase starts to oxidise forming chromate and/or molybdate This potential, and/or the time for which it is applied, may be optimised to promote pre-dissolution of chromium nitnde and molybdenum - πch phases which are not particularly embntthng 4 Switch from the previous controlled potential or cuπent to a low, controlled anodising cuπent density (this may be in a range depending on the speed of response required, e g lOμA/cmf may be used) Measure the potential response for an appropπate peπod (e g about 5 minutes) 5 If there are no deleteπous phases, the potential nses almost linearly revealing a capacitor-like behaviour of the metal interface, before reaching a steady value, ca 0 75 volt versus a saturated calomel electrode, depending somewhat on the individual steel composition The slope of the potential versus time relationship will be almost independent of the steel composition At this upper potential, which may be called the transpassivity potential, the whole steel surface is undergoing selective chromium and molybdenum oxidation, and possibly oxygen evolution 6 If deleteπous phases are present, the potential-time relationship shows an inflection or region of reduced slope In extreme cases the potential becomes almost constant at a value far below the transpassivity potential 7 Data analysis can be via differentiation of the potential-time relation The presence of sigma phase is indicated by a minimum in the differential curve, or in any case a reduced slope The amount of sigma phase can be quantified by the potential reached after a given peπod of anodising, by the slope over a fixed interval of time, or alternatively some function that includes the duration and depth of the minimum in the differentiated curve Referπng to Figure 1, the five curves shown represent the vaπation in the anodising potential with time for a stainless steel sample treated for 1000 seconds at the five respective temperatures 650°C, 750°C, 850°C, 950°C and 1050°C These temperatures are shown in Figure 1 with the figures in brackets after the treatment temperatures representing toughness values in joules deπved using the Charpy test The mateπal tested contained some mtermetalhcs before heat treatment The results were deπved after immersing the samples in a 0 1 M Na 2 C0 3 alkali solution A pre- passivation anodising potential of 250mV was applied for 1 mmute and thereafter an anode cuπent of lOμA/cm" was applied It can be seen from Figure 1 that immediately after initiation of the lOμA/cm " cuπent in all cases the potential rose rapidly and then at a slower rate The sample which had been treated at 1050°C and therefore contained no lntermetal c particles exhibited a substantially linear increase in potential to a level slightly above 0 7 volts over a penod of slightly more than 200 seconds The samples which had been heated to 750°C and 850°C exhibited potentials which rose fairly rapidly to 0 4 volts but then rose only very slowly thereafter The samples which had been heated to 650° and 950° exhibited potentials which did πse to substantially the same level as the sample which had been heated to 1050°C but over a much longer peπod (approximately 500 seconds for the sample heated to 650°C and in excess of 600 seconds for the sample heated to 950°C) In fact, the 650° result represents the as received (I e somewhat sigmatised) condition Figures 2 to 6 illustrate the linear fit of the five curves shown m Figure 1 It will be seen that for the sample heated to 1050°C, the slope of the curve m the interval 100 seconds to 200 seconds is 1 77 This contrasts with figures for the samples treated at 650, 750, 850 and 950°C of 0 798, 0 0923, 0 158 and 0 561 respectively Thus simply by monitoπng the slope of the curves in the 100 to 200 second interval a clear distinction can be made as between samples that are substantially free of deleteπous phases as against samples containing significant amounts such as 1% of deleteπous phases The result illustrated for the sample heated to 750°C is a little surpnsmg as the curve is very close to that of the sample heated to 850°C whereas there is a substantial difference between the measured Charpy toughness values It may be that this is the result of the formation of mtermetalhc phases other than sigma phase, but nevertheless it will be appreciated that the illustrated results provide a very sensitive way of detecting circumstances in which mtermetalhc compounds are present It is believed that using different pre-passivation treatments will make it possible to achieve the pre-dissolution of these interfenng phases The above example of the invention may be performed using a fibre-tip penlike device containing two electrodes, connected to an analogue to digital card and computer The double cuπent-step procedure described can operate on the basis of a simple voltage measurement If an initial passivation cuπent was not applied, it would be necessary to use a potentiostat to give passivation at controlled potential The monitored anodising potential as illustrated m Figure 1 can be calibrated to represent a measure of sigma phase present in the sample The time per measurement may be about 5 minutes but could be reduced to for example 1 or 2 minutes Different pre-passivation treatments and cuπents can be used to give two different outputs, that is one indicating total mtermetalhc content (relevant to coπosion as well as embnttlement) or embπttlmg phases only It is possible to detect when the sigma phase begins to oxidise Having done so, it is possible to apply a double potentiostatic procedure, that is step to just below the sigma oxidation point, then step above that potential and quantify the current-time relation that results An electnc charge density value can be extracted and correlated with the amount of sigma phase There are disadvantages with such an approach however as in different superduplex steels it appears that mtermetalhc phases oxidise at different potentials In Figure 1 the Charpy data given in brackets (m joules) indicate a lack of coπelation with the electrochemistry for the lower temperature heat treatments In the case of the 650°C treatment, this is because the as-received mateπal already had an impaired performance This is shown in Figure 8 which plots the results with the as- received matenal and after heat treatment to 1050°C and 750°C Essentially, treatment at 650°C for 1000s did not change the properties of the already degraded mateπal In the case of the 750°C treatment, the electrochemical degradation evident m Figure 1 is real and must result from precipitation of either an mtermetalhc phase or a nitπde At first it was thought that this was an artefact of pπor mtermetalhc content, but the sample solutionised at 1050°C and re-heated at 750°C as represented in Figure 8 also showed a very low potential Since only sigma phase is formed at 950°C, the response shown in Figure 1 for this treatment may be considered to be a "pure" sigma response (although it is possible that the prior exposure caused some other phase or phases to form). Clearly, a highly reactive phase or phases is formed at lower temperatures. TTT diagrams for similar steels indicate that chi (χ) and chromium nitride can both form in applicable time scales at 750 or 850°C. Such highly reactive phases compromise the ability of the test to quantify sigma phase. However, results obtained on the sample treated at 1050°C and then at 750°C are shown in Figure 9. This shows three clear potential aπests, with the highest aπest coπesponding to the single (sigma) aπest seen (in Figure 1) for the sample treated at 950°C. This means that suitable further development of the test can correct for the reactivity of the chi and nitride phases, leaving only the sigma response. Alternatively, should it be the case that chi is equally damaging as sigma, then only the nitride could be removed from the response. It is believed that the test will perform better on real components than on isothermally heated material where the low temperature phases are emphasised. In any case, the test provides a fast way to certify that a material is free from all intermetallic phases. This represents an important limiting case. It is estimated that the sensitivity of this technique is 0.1% for the low temperature phases and 0.5% for sigma. A second set of data has been generated using the procedure in accordance with the present invention (also refeπed to herein as the carbonate test). The test material used to obtain the second set of data was Zeron 100 obtained from Weir Materials (0.017C, 0.24Si, 0.54Mn, 0.021P, 0.001S, 24.97Cr, 3.58Mo, 6.97Ni, 0.52Cu, 0.54W, 0.22N, balance Fe), and was in the form of 16 mm-diameter round bar. Samples of the bar were either used as-received, or heat-treated for 100, 300 or 1000 s in air at 675 or 825C. In each case the heat treatment was followed by water quenching. The microstructures of the specimens are illustrated in micrographs shown in figure 10 (the micrographs comprise cross-sections of the bars with electrolytic KOH etch). Figure 10a is an untreated specimen, figures 10b to lOd are specimens that were heat treated at 675C for 100s, 300s and 1000s respectively. Figures lOe to lOg are specimens that were heat treated at 825C for 100s, 300s and 1000s respectively. The Charpy impact energy of the specimens was tested at -50C. The resulting broken specimens were mounted on end in epoxy resin for electrochemical testing. A 1 cm 2 area of cross-section just behind the fracture plane of each broken specimen was exposed to the test solution. All the electrochemical testing was performed on surfaces polished to a 1 micron finish. For one test condition (825C, 100 s), three different specimens were tested electrochemically. These are designated A, B and C in the results discussed below. For other conditions only one specimen was tested electrochemically. Ferrite meter measurements were obtained for each specimen to provide a comparison with the procedure according to the present invention. The Ferrite meter readings are plotted against Charpy impact failure energy in figure 11. Each point on the graph shown in figure 11 is an average for three specimens that have undergone the same amount of heat treatment. Figure 11 demonstrates that the Ferrite meter measurements lack discrimination at lower levels of deleterious phase content (i.e. the right hand end of the graph). 500 ml of sodium carbonate solution was used for the carbonate tests used to obtain the second set of data. A platinum counter electrode and SCE reference electrode were used. The tests were carried out without any de-aeration and at room temperature except where stated. The carbonate test consisted of applying a passivation potential of 250 mV for 1 minute followed by an anodic cuπent in the range 10 to 50 μA/cm . The potential was monitored for 10 minutes. Figure 12 shows an example of raw data obtained using a relatively low cuπent of 10 μA, the potential at each test specimen being shown as a function of time. Figure 13 shows the equivalent data for 50 μA. The sensitivity of the carbonate test can be seen to vary significantly when different cuπents are used. Figure 14 is graph which plots the Charpy impact failure energy against the potential after 500 s. Each line of the graph coπesponds to a different cuπent in the range lOμA to 50μA. Each point on the graph represents a different test specimen, a point nearest the left hand end of the graph represents a specimen which has undergone heat treatment at 825C for 1000s, the second and third points on the line represent specimens which have undergone heat treatment at 825C for 500s and 300s respectively. The fourth point on the line represents a specimen which has undergone heat treatment at 675C for 1000s, the fifth and sixth points on the line represent specimens which have undergone heat treatment at 675C for 500s and 300s respectively. The final point on the line represents an untreated specimen. Figure 14 shows that the carbonate test gives a strong coπelation between the Charpy impact failure energy and the potential after 500s. Furthermore, a significant variation of the form of the coπelation may be obtained by using a different cuπent. The cuπent required for a given carbonate test may be selected on the basis of a required sensitivity. It can be seen from figure 14 that an appropriate cuπent may be chosen (for example 15μA or lOμA) such that the carbonate test provides a "threshold" type of evaluation where the potential response drops suddenly at a particular Charpy impact failure value. The results of the carbonate test may also be analysed using a graph which shows Charpy impact failure energy against the time taken to reach a particular potential (750mV), as shown in figure 15. In common with figure 14, each line in figure 15 represents a different current, and each point represents a different specimen. Figure 15 shows a strong coπelation between the time taken to reach a potential of 750m V and the Charpy impact failure energy. The correlation is close to being linear, demonstrating that interpretation of the data in this way may provide a substantially linear electrochemical measurement of the Charpy impact failure energy of a specimen. A significant variation of the form of the coπelation may be obtained by selection of a different cuπent. The cuπent required for a given carbonate test may be selected on the basis of a required sensitivity. As mentioned above, any technique that detects chromium or molybdenum depletion rather than sigma phase direct cannot provide an ideal measure of the presence of sigma phase. For example, steels heat-treated at 850°C and 950°C have about the same very bad mechanical properties, but the latter has much less chromium depletion. Nevertheless, the results obtained in accordance with the example of the present invention set out above show good coπelation with tests utilising the present invention to indicate the presence of chromium depleted regions. Such tests are described below with reference to Figures 16, 17 and 18. Referring to Figures 16 to 18, simple anodic galvanostatic treatments were applied in 5% NaCl solution to freshly abraded surfaces of Zeron 100 steel, initially free of intermetallics and isothermally treated at 650, 850 and 1050°C. The samples were air-cooled and, in consequence, a small amount of sigma phase was formed in the 1050°C sample. Cuπent densities of 10, 50 and 500μA/cm 2 were used. Each test was carried out twice at the two lower cuπents. Figure 16 shows the response at a cuπent density of 500μA/cm 2 for four conditions, that is solution-annealed (control) 650°C, 850°C and 1050°C. Figures 17 and 18 show similar data for 50 and 10μA/cm 2 respectively. Potentials shown are on the SCE scale; a sustained potential around +1V indicates uniform transpassive dissolution, not pit-like localised coπosion. The following features are of note: • At the highest cuπent (Figure 16), the 850°C sample showed sustained pit-like coπosion along chromium-depleted zones that were connected in three dimensions, whereas the 650°C sample had Cr depletion of limited extent (this is probably due to some phase other than sigma, which would not form at this temperature) and thus showed only a brief potential aπest around 0.65V before going transpassive at 1.1 V. A small difference is evident between the control and the 1050°C sample • At the lowest cuπent (Figure 18), the 650°C, 850°C and 1050°C samples all showed pit-like behaviour for the entire 300s test time. Probably this means that the Cr depleted zones at 650°C, though finite, were not totally consumed during the test. The reproducibility was good as indicated. • At the intermediate cuπent (Figure 17), the 850°C and 1050°C samples again showed sustained pit-like coπosion but the 650°C sample showed a reproducible jump in potential, indicating that some kind of depleted zone was being totally consumed. However, there may have been another type of depleted zone, as the potential stayed at a low value thereafter. • For the lower cuπents, the 650 sample showed the lowest potential and thus the easiest pit-like coπosion, as expected from the deeper chromium depletion that occurs at lower temperatures. Clearly no coπelation with impact properties can be expected from such data, and none was attempted. However, the results could be used to certify material as entirely free from intermetallics or nitrides. There is a preliminary indication that different types of Cr-depleted zones are present and could in principle be separated. This is a vast improvement on the conventional EPR-type reactivation test, and extremely easy to carry out, given that all that is required is a one-step constant- cuπent anodising treatment, using a low cuπent density and a sodium chloride solution (e.g. 5%). The steel will not show pitting coπosion at ambient temperature if there is no chromium depletion, so if there is no sigma phase the potential rises up to the applicable transpassivity potential and stays constant. However the chromium depleted zones do show pit-like coπosion. Thus in summary, if chromium depletion is present, the potential either reaches a constant value far below the transpassivity potential (for a severe case where the chromium-depleted zones are connected throughout the microstructure) or shows an aπest at an intermediate potential before rising to the transpassivity potential (for a case where there are finite chromium- depleted zones connected to the steel surface). It is possible to avoid a pre-passivating electrochemical step by pre- passivating the steel chemically using an oxidising solution that establishes a constant redox potential, such as feπo/ferricyanide, or even an uncontrolled oxidising solution such as nitric acid. This would reduce the procedure to the application of a simple constant cuπent in a single step. However, pre-passivation is prefeπed. Figure 19 illustrates results for samples heated to 1050°C, 750°C and 850°C after the application of a relatively high pre-passivation potential of 0.4 volts for 5 minutes. This shows that the 750°C results are better in relation to mechanical properties than in the test represented in Figure 1 , presumably because some phases other than sigma are dissolved out by the passivation process. The present invention was used to indicate the presence of chromium depleted regions in a further set of specimens (the test is refeπed to hereafter as the chloride test). The chloride test was carried out using the specimens that were used to obtain the second set of data (i.e. the Zeron 100 specimens described above). 500 ml of sodium chloride solution was used for each test. A platinum counter electrode and SCE reference electrode were used. The tests were carried out without any de-aeration and at room temperature except where stated. The chloride test consisted of simply applying an anodic cuπent, generally 10 μA/cm , for 10 minutes. The test solution was generally 5% NaCl but the concentration was increased in a few tests. The temperature was increased up to 50C in some tests. Figure 20 shows raw data for room temperature chloride testing of the specimens. Only the specimen that has been heat treated at 825C for 1000s shows a persistent low potential indicative of localised coπosion due to solute depletion zones in the alloy. Increasing the temperature at which chloride testing was carried out gave a much better discrimination between samples as shown in figure 21. Two distinct types of behaviour are seen in figure 21. The specimens that had been heat treated at 875C for 1000s or 300s give a sustained low potential, lower than zero volts. The three specimens that had been heat treated at 825C for 100s (specimens A, B and C) give a sustained potential around 500 mV, which is due to repeated initiation and repassivation of small corrosion events. Finally, the specimens that had been heat treated at 623C show only a small degradation in the potential response compared with the untreated (as-received) specimen. Using a higher chloride concentration at room temperature provided a minor improvement of the discrimination between samples, as shown in figure 22, but the improvement was not as significant as that which was provided by increasing the temperature. Figure 23 shows the coπelation between the Charpy impact failure energy and the potential after 500s for specimens tested at a range of different temperatures. Each line on the graph represents a different testing temperature. Each point on the graph represents a sample that has been subjected to a specific heat treatment (the aπangement of the points is the same as that described above in relation to figure 14). It can be seen from figure 23 that the coπelation between Charpy impact failure energy and the potential after 500s becomes increasingly linear as the temperature is increased. At 50C a substantially linear coπelation with impact properties is obtained. This coπelation could be tuned still further if required; the important point is that moderate degrees of deleterious phase precipitation are quantifiable and can be separated from the most severe conditions. Sodium bromide may be used in place of, or in combination with, sodium chloride. The test was carried out with a 0.85 molar sodium bromide solution using the Zeron 100 specimens described above. The test was carried out at room temperature, using a current of 10μA cm 2 . Figure 24 shows the variation of potential for each Zeron 100 specimen from 0 to 500 seconds. Good discrimination between the samples is provided by the test. The potential of the control sample increases smoothly over time and tends towards a stable value of around IV. The potential of the heat treated samples initially increased at the same rate as the control sample, but then increased at a lower rate and generally stabilised at a range of values significantly below IV. The rate of increase of potential and the final potential are seen to have a close dependency upon the heat treatment applied to the samples. Figure 25 shows the potential at 500s for each sample, plotted against the Charpy failure energies of the samples. It can be seen from figure 25 that there is a strong coπelation between the potential at 500s and the Charpy failure energy, the coπelation being almost linear except for very low or very high Charpy failure energies. The sodium bromide test is particularly advantageous because it provides good discrimination between samples at room temperature (a similar level of discrimination to that provided by the sodium chloride test at 50C). The bromide solution does more than just reproduce the chloride result at a lower temperature. The chloride test is very sensitive to molybdenum, whereas the bromide test detects chromium depletion (molybdenum is dissolved much more easily in the presence of bromide ions). This means that there will be instances in which both tests may be used to provide complementary information. One example where both test are advantageous is where a particular deleterious phase (chi phase) has higher molybdenum level than another (sigma), so that the region around each chi particle is more depleted in molybdenum than the region around each sigma particle. chloride test responds differently to sigma and chi because chloride is very sensitive to molybdenum, while the bromide test will just detect the chromium depletion which is about the same for sigma and chi, providing complementary information. If sigma and chi are equally deleterious to mechanical properties, then the bromide test will give a better relationship with mechanical properties. The chloride and bromide solutions may be mixed together to provide a single test. This is advantageous because the bromide will tend to eliminate the beneficial effect of alloyed molybdenum on coπosion. Consequently, a mixed chloride and bromide solution test may be carried out at room temperature. The mixed solution test can be tuned to provide intermediate discrimination, and emphasise particular levels of sigmatisation. The mixing could be tuned to emphasise the relative effects of chi phase (which causes heavy molybdenum depletion) and sigma phase (which does not cause significant molybdenum depletion) by selecting the relative proportions of chloride and bromide.

Claims: CLAIMS 1 A method for assessing the presence of undesirable phases m a steel sample, wherein the sample is passivated, an anodising cuπent is applied to the sample such that an anodising potential applied to the sample increases over time, and the rate of increase in the anodising potential is monitored, a relatively low rate of increase in the anodising potential indicating the presence of undesirable phases 2 A method according to claim 1, wherein the rate of increase is monitored by monitoπng the slope of a curve representing the vanation of anodising potential with time 3 A method according to claim 1, wherein the rate of increase is monitored by measuπng the anodising potential a predetermined time after the initiation of the anodising cuπent 4 A method according to claim 1, wherein the rate of increase is monitored by measuπng the time taken for the anodising potential to reach a predetermined potential 5 A method according to any preceding claim, wherein the sample is immersed in a solution 6 A method according to claim 5, wherein the solution is an alkali solution which acts to passivate the sample 7 A method according to claim 6, wherein the solution is a carbonate solution 8 A method according to any preceding claim, wherein the sample is passivated by applying an anodising cuπent to the sample 9 A method according to claim 5, wherein the solution is a halogen solution 10. A method according to claim 9, wherein the solution is a chloride solution. 11. A method according to claim 10, wherein the concentration of chloride in the chloride solution is greater than 5%. 12. A method according to claim 10 or claim 11 , wherein the method is performed at a temperature greater than 20 Celsius, the temperature being selected to provide a required resolution of assessment. 13. A method according to claim 12, wherein the method is performed at a temperature greater than 30 Celsius. 14. A method according to claim 13, wherein the method is performed at a temperature greater than 40 Celsius. 15. A method according to claim 9, wherein the solution is a bromide solution. 16. A method according to claim 14, wherein the bromide solution is between 0.7 mole and 1.0 moles. 17. A method according to claim 16, wherein the bromide solution is around 0.85 moles. 18. A method according to any of claims 9 to 17, wherein the solution is a chloride and bromide solution. 19. A method according to claim 18, wherein the relative proportions of chloride and bromide are selected to provide information regarding different phases. 20. A method according to any preceding claim, wherein the anodising cuπent is selected to provide a required resolution of assessment. 21. A method according to claim 20, wherein the anodising current is less than 50μA. 22. A method according to claim 21, wherein the anodising current is less than 35μA. 23. A method according to claim 22, wherein the anodising cuπent is 15μA or less. 24. A method substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: PROFESSOR ROGER CHARLES NEWMAN
Applicants: CAPCIS LTD, ROGER CHARLES NEWMAN PROF, CAPCIS LIMITED, PROFESSOR ROGER CHARLES NEWMAN
Application year: 2001
Main classification: G01N
IPC codes: G01N27, G01N33