Platinum Metals In Stainless Steels
Platinum Metals In Stainless Steels
A Review of Corrosion and Mechanical Properties
The range of iron-based materials known as stainless steels are used throughout the world to manufacture products requiring special resistance to corrosive environments. The corrosion resistance of these materials depends largely upon the chromium content, which is usually between 10 and 30 per cent of the alloy constitution. Of equal significance is the ability to vary the structure of the alloy between ferritic, austenitic and martensitic, by control of the type and level of additional elements. There are, however, many corrosion limiting factors which restrict the use of each type of steel, and the addition of noble metals to the iron-base constitution, to improve this situation, has been considered. This article summarises most of the published work on the effects of platinum group metals on the corrosion and mechanical properties of stainless steels, but excludes investigations of non-stainless steels.
Two previous reviews of the effects of adding platinum group metals to iron-based alloys have been published in this Journal previously (11, 2). Information from these sources has been assessed and, where appropriate, included here for the sake of completeness.
Major element compositions of the steels reviewed here are generally self-evident, especially those under the Soviet Designation system described later. The compositions of some of the steels referred to in the text are not obvious, however, and these are given below, in the Appendix.
Much of the work featured in this review was originally published in the Russian language, and Soviet alloy designations used in this review have been transliterated from the Russian rather than translated, in order to avoid confusion with Western alloys which are similar, but not identical. The explanation of these designations is based on information contained in the “Handbook of Soviet Alloy Compositions” 3 and a British Iron and Steel Industry publication 4, while a complete description of steel and alloy compositions is given in a Soviet standard 5.
|Steel designation||Composition, per cent||Reference|
|Fe||Cr||Si (max.)||Mn (max.)||C (max.)||Other||Noble metal|
In Soviet alloy steel designations, the following symbols represent the following elements:
When the percentage of the element is not greater than about 1 per cent the letter for the element is not followed by a number. If the amount of the element is greater than 1 per cent, a number representing the content is placed to the right of the symbol, for example 4 per cent nickel is represented by N4.
The average carbon content is shown to the left of the letters, as hundredths of one per cent. In the case of a very low carbon content (less than 0.08 per cent), the numeral “o” is placed before the letters. The letter A appended to these designations denotes narrower composition limits, and low sulphur and phosphorus contents, and must not be confused with the symbol A representing nitrogen. Frequently, the letter L is used as a suffix to denote a cast steel.
Several groups of steel designations carry a prefixed letter which indicates a particular purpose or characteristic, such as, A- free machining, E- magnetic, Zh- straight chromium stainless, R- high-speed, Sh- ball-bearing, E-electrical and Ya- chrome-nickel stainless. The numbers following these letters are not normally indicative of the actual composition.
Platinum Metals-Modified Steels in Sulphuric Acid Solutions
Many of the investigations carried out on platinum group metals-modified steels have been done under the auspices of two notable Russian scientists, N. D. Tomashov and G. P. Chernova. Much of the work consisted of simple gravimetric analysis, but some important electrochemical studies of the corrosion behaviour of platinum group metals-modified steels were also made.
Hoar 1 quotes early work by Tomashov and his colleagues (6-12) on the beneficial effects of additions of platinum and palladium to a commercial 18 per cent chromium-9 per cent nickel steel when subjected to various concentrations of sulphuric acid at room temperature. Similar effects have been recorded by Tomashov and Chernova for Kh27,OKh25T, Kh28TL, Kh25, Kh25M2 and a plasma melted Cr-Ni-Mn-Mo steel. A summary of these results is given in Figure 1.
In the same way Tomashov, Chernova and Volkov 13 noted that a 24Cr-6Ni steel became passive with the addition of 0.5 per cent palladium when exposed to 10 per cent sulphuric acid at temperatures up to 100°C, while a 0.2 per cent palladium addition to Fe-40Cr alloy confers resistance to 40 per cent sulphuric acid at 100°C 14. The steel Kh25 with and without 0.7 per cent palladium was tested in 30 per cent sulphuric acid by Tomashov and Chernova 10 and its corrosion monitored by the evolution of hydrogen. At the same time the potential of the sample was recorded, Figure 2.
From these results it was shown that the initial corrosion rate increases with the palladium addition, but diminishes rapidly after a critical period. The sample potential showed corresponding behaviour. The mechanism proposed was that on initial immersion in a non-oxidising medium the corrosion reaction, that is the anodic reaction M → M+ + e-, is sustained by the cathodic reaction H+ + e-→ ½H2, the rate of which is determined by the hydrogen over-potential at the surface of the test piece. For surfaces with a high hydrogen over-potential, that is surfaces which are slow to discharge hydrogen, the rate of the cathodic reaction (hydrogen evolution) will govern the corrosion reaction (metal dissolution), the result being steady state dissolution of the material.
With noble metal-containing alloys, however, the hydrogen over-potential of the original surface is reduced and enhanced corrosion occurs. As dissolution of the less stable components occurs, the noble metal accumulates as discrete particles on the surface where the hydrogen over-potential is very low, and hydrogen evolution occurs at these sites. As the over-potential is low, the equilibrium H+ + e-→½H2 is shifted far to the right, and the corresponding potential shift is sufficient to take the sample into a passive regime with an accompanying reduction in the corrosion rate. The effect of rapid corrosion followed by passivation is con-sistendy seen in non-oxidising acid media, as shown in Figure 3, and Tomashov claims that the majority of the weight loss observed from Kh25 and Kh24M2 steels containing 0.3 per cent palladium was due to the initial corrosion effect 15. After passivation the maximum weight loss observed was 0.3 g/m2.h indicating significant passivation.
The variation of sample potential during corrosion can be complex, especially in higher alloy steels. Tomashov plotted the variation of sample potential of Fe25Cr6Ni steel alloyed with 0.1, 0.2 and 0.5 per cent palladium in 10 per cent sulphuric at 100°C 13. The corrosion of the base alloy occurred with regular potential variation between -0.26 V and -0.12 V, the corrosion potential of Kh25 steel being determined at -0.26 V. The shift to more noble potentials was thought to be due to the accumulation of nickel on the surface of the steel as corrosion proceeded. Due to the high nickel dissolution rate, however, the potential of the steel did not reach the critical passivation potential, and the steel remained active. The palladium-containing steels, however, show modified potential/time traces. The initial potential arrest at -0.12 V for the 0.1 per cent palladium steel was again thought to be due to nickel accumulation. After dissolution of this surface, the active corrosion period which followed presumably allowed accumulation of sufficient palladium to maintain the potential at -0.12 V until partial passivation (+0.01 to +0.03 V) took place at a later stage. On higher (0.2 per cent) palladium content steels, palladium accumulates more rapidly, shortening the arrest times and eliminating reactivation periods. The value of the final potential was close to the reversible hydrogen potential on palladium. This undoubtedly is due to the adsorption of hydrogen on the surface accumulation of palladium. The 0.5 per cent palladium addition causes rapid displacement of the potential to a fully passive regime. In this case it is clear that hydrogen evolution cannot be the only cathodic process occurring—the potential is displaced beyond the reversible potential—and oxygen depolarisation must be assumed. It is conceivable that for the lower palladium additions the rate of accumulation of palladium is lower than the rate of hydrogen adsorption. In the case of the 0.5 per cent addition the palladium must accumulate at the surface faster than the hydrogen can adsorb, and the alternative cathodic reaction can be sustained with the associated potential shift.
In a higher acid solution, (20 per cent) no steel passivates fully, although increasing the palladium content reduces the time to partial passivation, even though the final corrosion potentials (and hence corrosion rates) are similar for all the steels tested, Table I.
In hot, 90°C, sulphuric acid solutions of various strengths, Tomashov again followed sample potential during active corrosion 16, Figure 4. The alloy tested was a Fe27Cr8Ni2MoMnN steel with 0.2 per cent Pd. In the light of previous comments it would appear that at 20 per cent acid solution a partial passivation occurs due perhaps to the accumulation of molybdenum or manganese on the surface, which would possibly be superseded by passivation due to palladium accumulation. The 40 per cent solution is too severe and no passivation is possible due to a rapid dissolution rate. Between these extremes the sample potential moves rapidly to palladium agglomeration-induced passivation. Results of similar work by Tomashov 15 on very high purity ferritic steels Kh25 and Kh25M2 (2 per cent molybdenum) with 0.3 per cent palladium additions are shown in Figure 5. Again, the intermediate regime (in this case temperature) results in deeper passivation of the molybdenum bearing steel. Molybdenum, it is claimed, hinders the passivating effect of palladium (the steel Kh25M2Pdo.3 is only passive up to 50°C). However, it is reported that for passive Mo-Pd steels the corrosion losses are 10 and 20 times less than for similar, simple palladium-containing steels. Molybdenum in palladium-containing steels, when tested in sulphuric acid, reportedly “narrows” the effective acid concentration and temperature band in which the steel will passivate, but also deepens passivation characteristics.
Earlier work by Tomashov compares the performances of three related alloys, containing 3 per cent platinum and 3 per cent palladium against the base alloy, in 20 per cent and 30 per cent sulphuric at 50°C with respect to their composition and passivation potentials 11, Table II. It is apparent that the corrosion rate of the high nickel alloy is greatly exacerbated by noble metal additions, while that of the intermediate nickel alloy is largely unaltered. The simple chromium alloy, however, is very deeply passivated. The explanation for this effect was that the passivation potential for Kh18N60 is more positive than that for hydrogen ion depolarisation on palladium/platinum. Consequently, unless an alternative cathodic reaction that would sustain the required potential shift to passivation is available (that is, oxygen reduction), no passivation is possible. However, the enhanced hydrogen reaction rate due to the cathodic additions stimulates the dissolution (corrosion) of the alloy as shown in Figure 5. Similarly, the alloy Kh27 passivates at -0.15 V (S.H.E.), well within the capability of the enhanced hydrogen reaction, while the 9 per cent nickel alloy only just achieved passivation.
Electron microscopy studies of corroded samples of 0KI125T steel with additions of 0.1, 0.2 and 0.5 per cent palladium were undertaken by Tomashov 17. The samples were cathodically activated in 10 per cent sulphuric acid at 25, 50 and 100°C then allowed to corrode without external application of potential. Single and two-stage replicas were taken of the samples after various exposure times, and examined. At room temperature (25°C) both the 0.2 per cent and 0.5 per cent palladium steels showed similar effects, in that roughly spherical particles of palladium (20 to 60 nm diameter) were seen to form and became more numerous, but not larger, with time. The low palladium steel, however, exhibited a limited number of large palladium particles (approximately 1 μm diameter) which were found chiefly at grain boundary sites. At higher temperatures the palladium accumulation behaviour of the higher palladium content steels altered little, while the 0. 1 per cent palladium steel showed a 2 to 3 times decrease in palladium particle size at 50°C, and a substantial decrease at 100°C.
The nucleation of palladium particles occurs at a definite surface supersaturation. Consequently, the higher palladium steels would be expected to nucleate more readily and form more palladium particles of similar size, while at low palladium concentrations the nucleation rate would be somewhat less and the tendency, as was observed, would be to form fewer larger particles. The effect of temperature was to increase the rate of dissolution, increasing the rate of surface enrichment in palladium so allowing more rapid nucleation of particles and hence, a finer particle size.
A simple empirical formula which is based on the corrosion behaviour of the 0.5 per cent palladium steel was presented:
where Q = weight loss in g/cm2, n = per cent palladium addition and A = reaction coefficient.
For similar palladium accumulation morphology, the equation is an excellent fit, hence the 0.5 per cent and 0.2 per cent palladium steels follow the stated relationship at all temperatures, as does the 0.1 per cent palladium steel at ioo°C. In the low palladium steel, however, as the palladium precipitates coarsen with decreasing temperature so the discrepancy between predicted and actual performance increases, presumably due to the lower palladium surface area presented to support the hydrogen evolution reaction which is necessary to move the steel nearer to passivation.
Tomashov extended original work in order to investigate palladium additions up to 0.5 per cent in a range of steels, all of which were plasma melted to increase their nitrogen content to approximately 1 per cent 16. The palladium additions did not affect the microstructure of any of these steels, but hindered activation of the samples (by contact with zinc) for subsequent corrosion testing. Weight loss data in 20, 30 and 40 per cent sulphuric acid at 20 and 100°C are shown in Figure 6, while Figure 7 and Table III show the relationship between corrosion rate, auto-passivation time and palladium content. The higher palladium content steels reduce both the corrosion rate and the auto-passivation time.
The potentials of the alloys in Figure 7 were plotted as corrosion proceeded and showed a rapid rise of potential which corresponded to the initial rapid corrosion rate of the palladium alloys, Figure 8.
Biefer investigated the performance of the commercial ferritic stainless steel AISI 430 with several element additions 19, one being palladium, in the range o to 2 per cent. The effect of the additions was noted primarily on the anodic and cathodic polarisation curves of the alloys in 1N H2SO4 at room temperature.
Palladium has a noticeable effect on cathodic polarisation characteristics. The lower level additions tend to flatten the Tafel portion of the curve, while the higher level additions tend to steepen and displace it, so the intersection of this and the anodic curve—that is, where the electrochemical equilibrium between the two reactions lies—is well into the region of passivity. The subsequent corrosion current, icorr, is therefore reduced to very low levels. The deleterious effect of low level additions, and the highly beneficial effects of higher level palladium additions are clearly shown in Figure 9. By testing in sulphuric acid environments a map of passive/active behaviour of the alloys can be drawn, Figure 10.
Clearly, the addition of 0.99 per cent palladium is effective in inducing passivity at high and low acid concentrations, the base alloy showing rapid dissolution under all conditions. Passivity was arbitrarily defined as a positive corrosion potential, and consequently, as passivadon can occur at negative potentials, the passive regions are probably much broader. On initial immersion in the acid the potential was found to be negative, changing, in some cases, over 1 8 hours to a positive potential. This finding is in accordance with the observations of other workers, and corresponds to the formulation of palladium microcathodes on the surface as corrosion proceeds.
Biefer claims that the AISI 430 steel to which 0.99 per cent palladium had been added shows superior performance in concentrated sulphuric acid at high temperatures than the most highly alloyed austenitic steels, although he acknowledges the danger in comparing data from different test procedures 19.
The steels were also similarly evaluated in 1N H2SO4+ 0.5N NaCl solution. The base alloy exhibits breakdown of the passive film by pitting in chloride solutions.
Palladium additions cause large increases in the exchange current density (io) and some decrease in (icpp), where cpp = critical passivation potential, but not enough to cause spontaneous passivation. Biefer goes on to suggest that by modifying the cathodic processes by the addition of palladium and the anodic process by molybdenum addition, for example, substantial improvement in corrosion resistance may be expected. This theory was investigated in later work by Agarwala and Biefer 20. Alloy compositions are shown in Table IV with polarisation results in Table V.
|Steel number**||Alloying elements, per cent||In 1N H2O4||In 1N H2SO4 + 0.5N NaCl|
|Ecpp'. mV||icpp, μ A/cm2||Ecpp, mV||icpp, μ A/cm2|
|J-8-2||1.89V + 1.06Pd||-500||2,400||-||-|
See Table IV for steel compositions
As in previous work, active/passive maps in temperature/sulphuric acid concentration regimes were plotted for various alloy combinations. The combined addition of palladium and molybdenum passivates the steel over a wider range of dilute acid concentrations than the palladium addition alone, although molybdenum would appear to be detrimental to the resistance of the alloy to corrosion in concentrated acid. The addition of molybdenum does, however, allow the level of palladium to be reduced and still achieve passivity in dilute sulphuric acid. Additions of tungsten and vanadium with palladium have variable effects and seem to be susceptible to low acid concentrations in the range 40 to 70°C. Corrosion resistance in concentrated acid is also inferior to that of a 0.99 per cent palladium alloy.
Results of tests in 1N H2SO4 with various percentages of NaCl are given and in general it was noted that with the exception of the 0.19Pd-3.07Mo steel, all the palladium bearing steels spontaneously passivated in 1N H2SO4 + 0.5N NaCl, whereas those containing single element additions did not. It is planned to conclude Dr. McGill’s paper in the next issue of this journal.
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M. A Streicher, Platinum Metals Rev., 1977, 21, ( 2 ), 51
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“BISI 2000”, British Iron and Steel Industry Translation Service Publication
“Soviet Steel Standards”, COST 4543-57 and 4632-51
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