Study of the Fe(II-III) hydroxysalts transformation according to the pH and the concentration of the ions present

Abstract

Sulphated green rust, GR (SO4 2- ), is one of the main corrosion products of carbon steel in marine environments. It is Fe (II)-Fe(III) hydroxylsalt in sheets, consisting of alternating layers of iron-hydroxide type Fe(OH)2 , loaded positively due to the presence of the cations Fe(III) and negative interlayers consisting of anions and water molecules. This compound is strongly associated with the metabolism of sulphate-reducing bacteria, and can also evolve under cathodic protection. Thus, recently, GR (CO3 2- ) has been detected in place of GR (SO4 2- ) on already corroded ordinary steel, newly subjected to cathodic protection. This presence is due to the pH and[SO4 2−] [HCO3 − ⁄ ] conditions imposed by the cathodic protection. In this paper, we chemically synthesize sulfated and carbonate green rust in a chlorinated medium; we then study their respective transformation according to the concentration [SO4 2- ] / [HCO3 - ] ratio and pH. Our results show that from a GR (SO4 2- ), GR (CO3 2- ) is formed from a pH ≥8.2 for [SO4 2- ] / [HCO3 - ] = 12 and without any change in pH for [SO4 2- ] / [HCO3 - ] <12. Whereas from GR (CO3 2- ), GR (SO4 2- ) is formed for [SO4 2- ] / [HCO3 - ] > 1 without any change in pH.