The Corrosion and Electrochemical Behaviour of Ductile Chromium Alloys
The Corrosion and Electrochemical Behaviour of Ductile Chromium Alloys
Beneficial Effects of Platinum Group Metal Additions
Until recently the brittleness of chromium and its alloys has limited their use in applications which require a high resistance to corrosion in oxidising environments. Now, minor additions of selected metals can produce alloys with satisfactory low temperature ductility and workability, but even these alloys are susceptible to corrosion in non-oxidising environments. Passivation and corrosion resistance in such conditions can be increased by cathodic additions of noble metals; this paper reports a study of the effects of such additions on the corrosion and electrochemical behaviour of ductile chromium alloys in non-oxidising environments.
The high corrosion resistance of chromium and its alloys in oxidising environments makes them promising construction materials for use in chemical, oil refining and other industries. Formerly their employment has been limited by their brittleness but in recent years minor additions of several metals have been made to chromium to produce alloys with adequate low temperature ductility and workability (1, 2). However, the extra opportunities created by these new alloys are still limited by the fact that chromium alloys are generally susceptible to corrosion in non-oxidising media such as hydrochloric and sulphuric acid solutions. It has already been shown (3, 4, 5) that the tendency to passivation, and the corrosion resistance of chromium in a non-oxidising acid environment, can be increased by alloying with certain noble metals. A more detailed study of the electrochemical behaviour of these alloys and its effect on their corrosion has now been made.
Ductile alloys with 0.1 to 0.4 weight per cent of ruthenium, osmium, or iridium or up to 1 per cent of rhenium were prepared at the Institute of Problems of Materials Science, Academy of Science of the Ukraine, Kiev. Their corrosion and electrochemical behaviour was studied in sulphuric acid of strengths ranging from 5 to 60 per cent and at temperatures from 25 to 100°C. The test specimens were in the form of cylinders pressed into Teflon holders so as to leave one end of the cylinder exposed to the electrolyte. The alloy surface was prepared for corrosion tests by polishing with carborundum paper and degreasing with acetone, prepared specimens were kept in a desiccator for three hours prior to testing. Metal losses were determined by accurate weighing. Electrochemical tests were carried out on freshly polished samples; in all tests free access air was allowed.
The data obtained in these tests are presented diagrammatically in Figure 1 as a function of temperature and sulphuric acid concentration. Boundary lines separating regions of active corrosion from regions where passivation occurs are shown for each alloy. Below and to the left of a line the passive oxide film is stable; above and to the right the alloy dissolves readily. The chromium-rhenium alloy has the smallest range of conditions under which the alloy remains passive, and the chromium-iridium alloy has the largest. The stability of the passive film is increased by increasing the noble metal concentration, as shown in Figure 1, for alloys with 0.2 and 0.4 per cent of ruthenium. Below and to the left of the dotted line A the alloy with 0.2 per cent ruthenium self-passivates even after it has been cathodically activated by the application of a potential of –0.175 volt for 10 seconds. In the shaded region between the dotted lines A and B this alloy remains active for a time after the cathodic treatment. However, below and to the left of line B passivation is immediate when the cathodic potential is removed.
The rates of dissolution of the chromium alloys containing ruthenium, osmium and iridium, were measured at various temperatures and sulphuric acid concentrations where the alloys were in the stable passive state. For each alloy similar corrosion rates were obtained under all the test conditions, as shown in Table I. A sample of chromium tested under the same conditions corroded rapidly and the rate of corrosion only began to decrease in the test with 60 per cent acid, this strength of sulphuric acid being appreciably oxidising. The noble metal alloys remained passive in all the tests; their corrosion resistance was several orders of magnitude higher than that of chromium, and increased in the order osmium, ruthenium, iridium. The rate of corrosion was also reduced by increasing of the noble metal concentration of the alloy.
|Sulphuric acid concentration per cent||Temperature °C||Pure chromium (standard)||Corrosion rate, mm per year*|
|Alloying element, and amount in weight per cent|
Corrosion tests made in boiling 5 per cent sulphuric acid, as reported in Table II, showed that corrosion rates for the passive noble metal alloys were virtually unchanged with time. Figure 2 shows the corrosion potentials measured against the standard hydrogen electrode. For all the alloys the potentials became constant after 5 hours in boiling 5 per cent sulphuric acid. The potentials for the ruthenium, osmium and iridium alloys remained in the passive region throughout the experiment. However the osmium alloy potential was close to zero volts and, as expected, the corrosion rate was higher than for the ruthenium and iridium containing alloys. Chromium itself, and an alloy containing rhenium, showed negative corrosion potentials and thus they were actively corroded under these test conditions. In conditions corresponding to the shaded region in Figure 1 the alloy with 0.2 per cent ruthenium showed special behaviour. After the termination of the cathodic prepulse, which consisted of −0.175 volt applied for 10, 15 or 20 seconds, the corrosion potential was at first positive, corresponding to regions b1, b2 and b3, respectively, in Figure 3. The potential then fell to reach the active dissolution potential—c1, c2, c3 in Figure 3—and then rose again to form a plateau—d1, d2, d3. Next the potential moved steeply, as indicated by f1, f2 and f3, to the passive region where each alloy eventually attained a stationary potential of about +0.6 volt.
The first positive regions on these potential-time curves is due to saturation of the noble metal component, ruthenium, with atomic hydrogen during the cathodic pre-treatment. Hydrogen is adsorbed by surface atoms or aggregates of atoms of the noble metal which have a low hydrogen overvoltage. After the cathodic pulse, the potential rises to approach that of the reversible hydrogen electrode—points b. However, if only small amounts of hydrogen are involved, desorption will prevent this potential being reached—b1 and b2. With sufficient adsorbed hydrogen the alloy does, in fact, reach the hydrogen potential, which in an acid environment at a hydrogen partial pressure less than one atmosphere may be calculated to be rather greater than zero volts (6).
After desorption of hydrogen the alloy surface activates, its potential rapidly decreasing from b to c. Increasing the cathodic polarisation time from 10 to 20 seconds causes the b loop to expand by a greater amount than expected. The reason is an increase of the noble metal component on the surface of the alloy during polarisation (7). If the alloy was polarised by short pulses, of two seconds duration or less, activation was incomplete and passivation was immediate after the pulse, as is shown by curve 4 in Figure 3. Pulses of 30 seconds or longer resulted in an appreciable accumulation of ruthenium on the alloy surface and the self-passivation occurred sooner after prepolarisation. In this case, after hydrogen desorption the activation peak was not observed, as shown by curve 5 in Figure 3. In experiments where the polarisation time was kept constant at 10 seconds and the cathodic potential was varied from −0.13 to −0.25 volt the potential-time curves obtained for the same 0.2 per cent alloy were similar to those in Figure 3. Pulses less negative than −0.13 volt did not activate the alloy since they are within the self-passivation region.
The effect of the ruthenium concentration of the alloy on the potential-time curves was also studied in 40 per cent sulphuric acid at 65°C. A cathodic prepulse of −0.175 volt for 10 seconds was used and the results are shown in Figure 4. Under these conditions chromium activation took place immediately without the prepulse. The 0.1 per cent ruthenium alloy reached the active dissolution potential in about 30 seconds without polarisation. Then, due to ruthenium accumulation on the surface, the potentail became more positive but the alloy did not completely passivate. The 0.2 per cent alloy activated only after polarisation, and then self-passivated after being active for a short time. The 0.3 and 0.4 per cent alloys immediately repassivated after the polarising pulse.
Minor amounts of chloride ions, 5 to 10 milligrams per litre, in the sulphuric acid solutions reduced the region of passivation stability for the 0.1 to 0.4 per cent ruthenium alloys, as shown in Figure 4. The addition of 10 milligrams of chloride ions per litre of 40 per cent sulphuric acid at 65°C promoted activation of the 0.3 per cent alloy; and 20 milligrams per litre was sufficient to activate the 0.4 per cent alloy and to prevent seif-passivation of the 0.2 per cent alloy.
Accumulation of the noble metal on the surface of the 0.1 to 0.4 per cent ruthenium alloys during active dissolution is indicated by the current-time curves obtained during cathodic polarisation at a constant potential of −0.175 volt. Changes in the size and direction of this current, as shown in Figure 5, reflect the kinetics of the electrochemical process. The oxide film on unalloyed chromium dissolved completely in 25 seconds. For alloys with 0.1 to 0.3 per cent ruthenium the current was cathodic initially, then it decreased rapidly and became anodic. After reaching an anodic maximum it decreased, eventually becoming cathodic again and finally reaching a constant value in the cathodic region. With increasing ruthenium content the anodic current appeared later and disappeared earlier; at the same time its maximum value decreased and occurred earlier and the stationary value of the final cathodic current increased.
These trends may be explained as follows. On immersion of the alloys in the solution the anodic dissolution of chromium is hampered by the passive atmospheric oxide film. At the same time, reduction of the hydrogen can occur on the noble metal causing a cathodic current. During activation the anodic process exceeds the cathodic, and the overall current is anodic. Dissolution of the alloy enriches the surface in ruthenium and the cathodic current rises to exceed the anodic current. The cathodic process is more efficient when the alloy has a higher ruthenium content initially, and the alloy with 0.4 per cent ruthenium shows a continuous increase of the cathodic current from the beginning of the test.
Weight loss measurements were included in the current-time curve experiments to determine corrosion as a function of time. As shown in the small graph inserted in Figure 5, the rate of dissolution of the alloys decreased with increasing ruthenium content, but the corrosion rates were constant for each alloy, the corrosion curves being straight lines passing through the origin. This leads to the conclusion that the ruthenium in the alloy slows the anodic dissolution of chromium, whereas the accumulation of ruthenium on the surface has no such effect.
This increase in ruthenium content on the surface of the alloys during chromium dissolution has been calculated from the weight losses and is shown in Table III. It has been assumed that all the ruthenium released by dissolution of the chromium remains on the surface of the alloy. As expected, the table shows that the ruthenium accumulated is proportional to the active dissolution time. It is not, however, proportional to the ruthenium content of the alloy in the range being considered because of the greater corrosion resistance of the richer alloys. This may reduce the amount of ruthenium accumulating on the surface of a rich alloy to a lower value than is observed for the less noble alloys.
|Polarisation time, seconds||Accumulation, expressed as atomic layers of ruthenium*|
|Ruthenium content of the alloy, in weight per cent|
Previously it has been shown (8, 9) that the noble metal forms separate islets on the surface of the alloy rather than a homogeneous layer. The surface coverage by such microcrystals is not high and we have estimated a value less than one per cent of the total surface. It can thus be understood why the noble metal does not affect the kinetics of active dissolution of the alloys.
It has been shown that ductile chromium alloys with additions of 0.1 to 0.4 per cent of osmium, ruthenium or iridium are stably passive for a wide range of temperatures and sulphuric acid concentrations, and they display high corrosion resistance.
In addition the self-passivation region for a particular alloy composition, and the tendency to repassivate, is reduced by the presence of chloride ions.
Finally, the rate of dissolution of chromium-ruthenium alloys in the active state decreases with increasing ruthenium content in the alloy. Ruthenium accumulating on the surface of the alloy during the corrosion process has no appreciable effect on the anodic dissolution process but does play a major role in shifting the corrosion potential positively, and thus in making the alloy passive and corrosion resistant.
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