Doctor of Philosophy
Dr. David W. Shoesmith
Ni-Cr-Mo (W) alloys, having an optimum concentration of Cr. Mo and W, are highly corrosion resistant alloys which do not fail by uniform corrosion in aggressive environments. However, when creviced at high applied potential and high temperatures, these materials can be susceptible to crevice corrosion. This study focuses on a family of seven commercial Ni-Cr-Mo (W) alloys containing various amounts of Cr, Mo and W.
Different electrochemical techniques were used to determine the characteristic potentials and temperature which indicate susceptibility to crevice corrosion, and to investigate the individual and synergistic roles of major and minor alloying elements. The corroded alloys were characterized by various surface characterization techniques to determine the effect of alloying elements on the inhibition of crevice corrosion.
The effects of Cr, Mo and W on the crevice corrosion of Ni-Cr-Mo (W) alloys were studied using the potentiodynamic-galvanostatic-potentiodynamic (PD-GS-PD) technique to measure film breakdown (Eb) and repassivation potentials (ER,CREV) as well as protection temperatures (TPROT) . As expected, Cr is the key element determining resistance to crevice initiation but a substantial Mo alloy content is required to achieve maximum film stability especially at temperatures > 60oC. Mo, not Cr, is the major element controlling crevice propagation and repassivation. If TPROT is accepted as the key indicator of an alloys overall resistance to crevice corrosion then the resistance increases in the order; 625 < C-4 < C-276 < C-22 ~ 59 ~ C-2000 < 686. More generally, this order could be written; High Cr-Low Mo < Low Cr – High Mo < High Cr-High Mo < High Cr-High (Mo + W). The individual influences of Mo and W appear to be inseparable and, while adding W improved the resistance, adding the equivalent amount of Mo could achieve the same improvement.
Crevice corroded Ni-Cr-Mo (W) surfaces were analyzed using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analyses, profilometry and Auger electron spectroscopy (AES). The mode of crevice propagation was found to be strongly dependent on the Mo (Mo + W) content of the alloy. At low contents of these elements deep local penetration occurred. At higher contents propagation took the form of interlinked pits along grain boundaries and only shallow pitting was observed on the grain surfaces. The depth of penetration at any individual grain boundary pit was limited by the accumulation of molybdates and tungstates. At very high Mo (Mo + W) contents, grain boundary pitting was eliminated and only a generally distributed shallow propagation occurred, consistent with the widespread distribution of molybdates and tungstates.
The influence of the minor alloying element, Cu, on the crevice corrosion of Ni-Cr-Mo alloys was investigated in chloride solutions at temperatures up to 105oC using the same electrochemical and surface analytical techniques. Cu did not have any measurable effect on passive film properties or on either the breakdown and repassivation potentials or the protection temperature. Galvanostatically controlled crevice corrosion experiments clearly demonstrated that Cu suppressed metastable breakdown events. Dynamic secondary ion mass spectrometry showed Cur accumulated in crevice corroded locations but could not confirm any influence of Cu on crevice propagation
The apparently anomalous behavior of these alloys in carbonate containing solutions was also investigated. In the presence of carbonate an unexpected anodic breakdown is observed in the otherwise passive potential region. In the passive region the film possesses the expected bilayer structure with a Cr (III)-dominated barrier layer containing mixed oxidation states of Mo and an outer dominantly-hydroxide layer. At more positive potentials the Cr/Mo content of the film decreases when bicarbonate is present and the alloy becomes covered by a thick (> 100 nm), and only partially protective, Ni(OH)2 layer. It was shown that the key feature leading to this depletion in Cr/Mo is the buffering of surface pH to > 8.6, when the surface deposition (or retention) of insoluble protective Mo(VI) species does not occur as would normally be expected if local acidic conditions, generated by Cr(VI) and Mo(VI) dissolution, were allowed to develop.
Mishra, Ajit K., "Crevice Corrosion Behavior of Ni-Cr-Mo-W Alloys in Aggressive Environment" (2013). Electronic Thesis and Dissertation Repository. 1721.