Effects of nitric acid passivation on pitting corrosion of stainless steels

Abstract

Through the pitting experiments for austenitic 316 and duplex 2205 stainless steel, the effects of nitric acid pre-treatments on the pitting behavior were investigated and more details on the pit growth were found and suggested. Samples of 316 stainless steel have been subjected to passivation treatments at ambient temperature for one hour in solutions of up to 50 wt % nitric acid. Pitting potentials of acid treated samples in 1 M NaCl at 70 oC were shown to vary with the concentration of the acid; increasing with concentration, up to 20 wt % and then decreasing as the acid concentration was further increased. There are two kinds ofbenefιt in pitting resistance from the surface pre-treatment: firstly, the removal of sulphide inclusions inhibits the initiation of pits and secondly, the enrichment of Cr in the passive film also makes the film more resistant to pit nucleation. Acid treatments removed MnS, but did not remove other kinds of inclusions, such as Ti carbides or Al oxides. Cr enrichment occurred in the passive film during acid treatment and reached a peak value for an acid concentration of 25 wt % and this was correlated with variations in the pitting potential. However, the main effect of acid treatment on improving pitting resistance is attributed to the removal of MnS rather than Cr enrichment in passive film. It is proposed that MnS inclusions are not only preferential sites for pit initiation, but that they also have a significant effect on pit propagation, probably through a combination of geometric and electro-chemical effects. That is, MnS inclusions create an initiation geometry favouring undercutting of the surface and subsequent formation of a pit cover. Also, the activating effect of dissolved sulphur species tends to produce etched pits rather than polished ones. Overall, data in this work suggests that acid treatments selectively remove MnS from the surface, and that this is done most efficiently by 20-25 % concentrations of nitric acid. MnS inclusions are the sites most likely to initiate pits at a relatively Iow potential and, when they are removed, pit initiation is displaced to higher potentials, where other initiation sites become available. Pits that can only initiate at high potentials then have an increased survival probability by virtue of that high potential. When nitric acid concentrations other than 20- 25 % are used, the removal ofMnS inclusions is Iess efficient, probably due to a decreased rate ofMnS dissolution relative to that in 20-25 % solutions. The sites most likely to retain some MnS after passivation treatments are also the most dangerous because they support initiation at relatively Iow potentials and produce pits with a high probability of becoming stable. The statistical results have shown that the transition from metastable pit growth to stable propagation is not linked in any simple way to either the rate at which metastable pits fbrm, or the number of pit sites on the specimen. A critical pit stability product (ifl) of 3 mA cm'1 can be calculated for open hemispherical pits in 316 stainless steel, but our experiments show that almost all pits grow with ia < 3 mA cm'1 because of the beneficial effect of the pit cover. The modified pit stability criterion was proposed, which takes account of the openness of pit cover. The development of the pit cover has been modelled and simulated using a finite element method based on that pit dissolution sharply depends on the pit solution chemistry. In the simulation, pits naturally develop a pit cover and become more open as they grow. Ecf,γ, the potential at which ia values in metastable pits increase abruptly, is possibly the lowest value at which any pit can reach ia = 3 mA cm1 and could be used as a more conservative estimate of pitting potentials. E pit for 316 stainless steels in 1 M NaCl decreased by 260 mV as the temperature increased from 20 0C to 70 oC. The metastable pitting rate and the pitting potential are also shifted by the same amount, as are the pit current density and pit stability product distributions. Hence, it is suggested that the decrease of E p,t is due to the increased stability of pit propagation, rather than any changes in the passive film. The metastable pitting rate of duplex stainless steel was quite Iow compared to that measured on316 stainless steel, suggesting that there are much Iess initiation sites. Different pit initiation sites appear more important in 2205 stainless steel, particularly phase boundaries between austenite and ferrite. Pit growth follows the same rules in 2205 as in 316 stainless steel, but a more concentrated pit chemistry is required to maintain pit growth. Consequently, stable pitting requires higher potentials in 2205 than in316.