Corrosion mechanism is the sequence of electrochemical and chemical steps by which a metal in contact with an environment is converted back to an oxidised form, and identifying it means establishing which step limits the rate and which species take part. For a passive metal in a halide solution the mechanism of interest is the local breakdown of the passive film: chloride adsorbs, forms a soluble complex with the cation, and thins the film at a point until the bare metal dissolves fast enough to sustain a pit, and a model for that sequence was proposed for passive stainless steel in acid chloride [1]. In commercial stainless steel the site is rarely random, since non-metallic inclusions at the surface provide the initiation point [2]. Thin electrolyte layers behave differently from bulk solution, and pits under a chloride droplet grow under a transport constraint set by the droplet geometry [3]. Once a pit is established its growth can be treated as a dissolution kinetics problem [4]. Additively manufactured material adds a further variable, because its microstructure differs from the wrought alloy [5].
- pitting corrosion
- passive film
- chloride ion
- pit initiation
- metastable pitting
- localised attack
- dissolution kinetics
References
- Zhang, P.; Wu, J.; Zhang, W.; Lu, X.; Wang, K.; A pitting mechanism for passive 304 stainless steel in sulphuric acid media containing chloride ions. Corrosion Science 1993, 34, 1343-1354, 10.1016/0010-938x(93)90091-t.
- Zheng, S.; Li, C.; Qi, Y.; Chen, L.; Chen, C.; Mechanism of (Mg,Al,Ca)-oxide inclusion-induced pitting corrosion in 316L stainless steel exposed to sulphur environments containing chloride ion. Corrosion Science 2013, 67, 20-31, 10.1016/j.corsci.2012.09.044.
- Tsutsumi, Y.; Nishikata, A.; Tsuru, T.; Pitting corrosion mechanism of Type 304 stainless steel under a droplet of chloride solutions. Corrosion Science 2007, 49, 1394-1407, 10.1016/j.corsci.2006.08.016.
- Saadawy, M.; Kinetics of Pitting Dissolution of Austenitic Stainless Steel 304 in Sodium Chloride Solution. ISRN Corrosion 2012, 2012, 1-5, 10.5402/2012/916367.
- Duan, Z.; Man, C.; Dong, C.; Cui, Z.; Kong, D.; wang, L.; Wang, X.; Pitting behavior of SLM 316L stainless steel exposed to chloride environments with different aggressiveness: Pitting mechanism induced by gas pores. Corrosion Science 2020, 167, 108520, 10.1016/j.corsci.2020.108520.