Electrochemical evaluation of the effect of different NaCl concentrations on low alloy- and stainless steels under corrosion and erosion-corrosion conditions

The main objective of this study was to assess the influence of salt concentration on the corrosion behaviour, including the role of hydrodynamic conditions, of two broad classes of ferrous engineering materials. These are comprised of alloys, typified by a low-alloy steel (UNS G43400) that corrodes actively in aqueous conditions and a range of passive-film-forming stainless steels (UNS S31600, UNS S15500 and UNS S32760). Corrosion monitoring employed electrochemical (potentiodynamic polarisation) techniques. Three concentrations of aerated sodium chloride were utilised: 0.05 wt% NaCl, 3.5 wt% NaCl and 10 wt% NaCl. In quiescent, liquid impingement and solid/liquid impingement conditions, the corrosion rate of the low-alloy steel was observed to peak at 3.5 wt% NaCl, followed by a reduction in 10 wt% NaCl solution. These findings expand the range of previously reported trends, focused on static conditions. Such corrosion rate/salinity trends were observed to be dictated by the progress of the anodic reaction rather than influence on the cathodic reaction. Detailed studies were undertaken using segmented specimens to facilitate comparisons of the influence of hydrodynamic variations on corrosion behavior; these revealed that such variations influence the corrosion rates of low-alloy steel to a much lesser extent than the effect of changes in salinity. For the stainless steels, in quiescent and flowing conditions, when surface passive films are stable, there was a constant increase in corrosion rate with salinity. In solid-liquid conditions, however, the periodic film-destruction/repassivation events resulted in a similar corrosion rate/salinity trend to that displayed by the low-alloy steel, but with a much larger effect of hydrodynamic conditions. Additonally, the study revealed an underlying influence of stainless steel composition that mirrored, to an extent, the corrosion behaviour in pitting/re-passivation situations