Influence of Mo alloying on pitting corrosion of stainless steels used as concrete reinforcement

Abstract

Corrosion of reinforcement steels, induced by chloride ions penetrating into the concrete, is the main cause of early damage, loss of serviceability and safety of reinforced concrete structures, which can be even more severe in the presence of concomitant concrete carbonation. In order to prevent reinforcement steel corrosion in highly aggressive alkaline environments, the use of stainless steels is becoming increasingly popular in coastal and marine constructions. Although widely used as an increasing corrosion resistance element in acidic environments, the influence of Mo addition on pitting corrosion resistance of stainless steels is not very clear in these conditions. Understanding Mo mechanism on corrosion resistance in alkaline media is hence of major importance, particularly for new lean grades with low Nickel and Molybdenum contents which presents a good balance between the properties required in these applications and the final cost of the material. In this work we will show the effect of Mo addition on pitting corrosion properties of austenitic, ferritic and duplex SS. A comparison between Mo content steels (alloys: 1.4404, 1.4113 and 1.4462) and very low molybdenum contents steels (alloys: 1.4301, 1.4016 and 1.4362) is done considering their pitting corrosion resistance (pitting potential E pit ) in different corrosion conditions. The results are discussed with respect to the influence of Mo addition on pitting behaviors for the different stainless steel rebar families in several aggressive media mainly in synthetic, chlorinated and carbonated solution reproducing the real concrete pore environments (pH10 solution with carbonates and chlorides ions). Keywords

Study of plasma nitriding and nitrocarburizing for higher corrosion resistance and hardness of 2205 duplex stainless steel

“…The reason why the ferrite phase did not pit can be attributed to the fact that it is rich in Cr and Mo (Table1). Our observation that ferrite is resistance to pitting is also supported by Mesquita et.al [37] in a similar study on this steel.…”

Corrosion resistance of chromium‐steel and stainless steel reinforcement in concrete

“…Molybdenum's effect on pitting resistance has been found by a number of studies to increase as the pH of the surrounding solution decreases, which explains why molybdenum‐bearing reinforcements (316LN and 2205) showed the same performance as 304 in alkaline environments, but a better performance in neutral environments (exposure to sea water). Molybdenum has proven to increase the resistance of reinforcing steel to pit formation, reduce the corrosion rate, encourage the repassivation process and further decrease the pit propagation when molybdenum oxides form on the pit walls . In any case, no evidence was found in the current study that associates a lower corrosion resistance with 304 steel in alkaline media.…”

“…The presence of magnesium, albeit to a limited percentage, may have a detrimental effect on pitting corrosion compared to nickel. [14,19,24,[27][28][29] Therefore, A1035 is expected to be less corrosion-resistant than stainless steel, and is only considered to have a medium corrosion resistance, useful in applications demanding medium resistance at a slightly higher cost. Conclusion in Ref.…”

“…This also agrees with the observations by Rondelli et al [27] Other authors have demonstrated that manganese sulfide inclusions, which may happen in cases where the pore solution is rich in sulphates, can adversely affect the properties of the passive film and substantially decrease the resistance of steel in concrete to pitting corrosion surrounding these inclusions. [19,29,30] Park and Kwon [28] also demonstrated that Mn facilitates the metal dissolution reaction by enhancing the activity of iron. Furthermore, the authors found that the Mn addition appears to suppress the passivation process by reducing the activity of Cr-adsorbed species, and hence significantly decreasing the repassivation rate.…”

“…[24,27,28] This is especially prevalent in cases where manganese sulfide inclusions prevail, which may occur in pore solutions rich in sulfates. [19,29,30] A review of the literature and the proposed mechanisms by which each alloying element plays a role in the pitting resistance of stainless steels clearly indicates that the approach typically followed in concrete research literature, referring to stainless steel reinforcements as one group having a similar chloride threshold, may yield misleading service-life predictions. This is due to the roles that different alloying elements play, and to the different alloy concentrations in stainless steel grades.…”