| dc.description.abstract | Crevice corrosion is a localized form of attack that occurs at shielded areas on metal surfaces exposed to certain specific corrosive environments. This type of corrosion occurs very frequently in engineering structures, particularly with threaded or riveted joints, gasket fittings, welded lap joints, and coiled or stacked sheets. The crevice corrosion susceptibility of a metal is dependent on the geometry of the crevice, chemical environment, and the metallurgical state of the material.
The aim of the present investigation is to study the influence of microstructure on the crevice corrosion behaviour of austenitic stainless steels in aqueous media containing chloride ions. Austenitic stainless steels have excellent uniform corrosion resistance but are highly susceptible to localized corrosion such as pitting, crevice corrosion, and stress corrosion cracking.
The steels chosen are AISI Type 304 (18Cr–10Ni), 316 (17Cr–12Ni–2Mo), and 310 (25Cr–20Ni). These were selected for the following reasons:
(i) For a given chromium content, an increase in nickel in austenitic stainless steels increases the stacking fault energy (SFE), which has considerable influence on the microstructure of the material.
(ii) These steels find important applications in nuclear and chemical industries, steam generator plants, and biomaterials.
(iii) They provide a basis for studying the influence of molybdenum addition (in 316).
(iv) They exhibit different microstructural characteristics under thermomechanical treatment.
As there is no universally accepted or standard test method for studying crevice corrosion susceptibility as a function of metallurgical variables, a crevice assembly was designed and fabricated for this purpose. The crevice corrosion resistance was measured using a potentiodynamic method in terms of the critical crevice potential (Ecc). The higher the susceptibility of the material, the more active (less noble) is the Ecc value.
The metallurgical factors considered in this study are: crystallographic texture, grain size, carbide precipitation (sensitization), dislocation arrangements, and non-metallic inclusions. Texture and dislocation arrangements depend on the SFE of the stainless steel and can also be varied by cold work. Grain size and carbide precipitation leading to sensitization can be controlled by heat treatment. The effect of non-metallic inclusions was studied using materials of different cleanliness.
Potentiodynamic as well as galvanostatic studies were carried out using the standardized crevice assembly in a neutral 0.5 N NaCl solution at ambient temperature. In the potentiodynamic test, the Ecc value was independent of scan rate in the range 5–100 mV/min, and a scan rate of 10 mV/min was chosen for the present study.
The crevice corrosion potential was measured on solution-annealed specimens as well as on specimens cold rolled to different thickness strains in the range 5–20%. Measurements were made on three perpendicular cross-sections to evaluate the effect of texture. Grain size variations were achieved by thermomechanical treatments, and specimens with different degrees of sensitization were prepared by soaking at 1023 K for varying times between 20 minutes and 24 hours, followed by air cooling to ambient temperature.
Texture in the specimens was measured using X-ray techniques. Integrated line intensities as well as pole figures were recorded on the specimen surface. The microstructure was examined using optical, transmission electron, and scanning electron microscopy techniques.
The crevice corrosion resistance of 316 stainless steel was better than that of 304 and 310 when tested on the rolling surface. This is attributed to the beneficial effect of molybdenum in 316. Also, 310 stainless steel showed lower resistance to crevice corrosion than 304. The inclusion content in the cleaner grades of the steels studied did not have any significant effect on crevice corrosion susceptibility.
The Ecc measured on the rolling surface of solution-annealed 316 and 310 steels was more noble than that obtained on the two perpendicular surfaces (transverse and cross-transverse). In solution-treated 304, however, all three sections of the specimen had similar Ecc values.
The crevice corrosion resistance of all three stainless steels decreased with cold work up to about 5–15%, and further increase had negligible effect. In the cold-rolled condition, all three steels showed more noble Ecc values on the rolling surface than on the other two sections. These results are interpreted in terms of the textures developed in the three steels. In the solution-annealed condition, 316 and 310 steels showed a {112}<111> type texture, while 304 showed a less ideal {112}<231> type texture. In the cold-worked state (20%), the texture in 304 and 310 was {011}<211>, and that in 316 was {112}<111>.
The differences in crevice corrosion behaviour are attributed essentially to variations in the crystallographic nature of the exposed surface. In the cold-worked condition, there could also be contributions from changes in grain size from one surface to another. The results indicate that the resistance to crevice corrosion improves when a less close-packed crystallographic plane is oriented parallel to the exposed surface. This improvement is caused by the development of a macro-galvanic couple between the crevice and non-crevice areas, the intensity of which is lower if a less close-packed plane is parallel to the surface.
A correlation between the crystallographic nature of the exposed surface, as obtained from texture data, and crevice corrosion resistance was established and supports the above interpretation.
The effects of grain size and carbide precipitation (sensitization) were studied in detail for 316 stainless steel. The Ecc value (resistance to crevice corrosion) decreased with a decrease in average grain diameter (d), following a linear relationship between Ecc and d¹². Sensitized samples, after crevice corrosion testing, showed intergranular attack inside the crevice, as against no such attack outside the crevice, and exhibited increased susceptibility to crevice corrosion. This is attributed to chromium depletion at the grain boundaries, which creates more active sites for the initiation of crevice corrosion. The increase in active grain boundary sites is also responsible for enhanced crevice corrosion in fine-grained materials.
To simulate the chemical and electrochemical changes occurring within the crevice during corrosion, 316 stainless steel specimens were exposed to solutions containing different concentrations of cations, chloride ions, and hydrogen ions (pH), and anodic polarization studies were conducted. It was observed that complete breakdown of passivity, leading to rapid crevice corrosion, occurs when the pH is less than 2.4 and the chloride ion concentration exceeds 2.0 N. This observation supports the contention that crevice corrosion occurs when the solution inside the crevice becomes aggressive and an active–passive cell forms between the crevice and non-crevice areas. | |
