FEASABILITY OF NICKEL SULPHIDE AS A CATHODE IN A GALVANIC COUPLED ANODIC PROTECTION SYSTEM FOR USE IN CONCENTRATED SULPHURIC ACID SYSTEMS

Abstract

<p>Anodic protection has shown to be a viable method for reducing corrosion rates of stainless steels over a wide range of temperatures and is used to protect equipment in H₂SO₄ manufacturing. While effective at controlling corrosion in H2SO4 manufacturing, Impressed Current Anodic Protection (ICAP) systems have shown to have a number of issues. They require a constant source of current to ensure reliable corrosion protection; are relatively complex systems and expensive to install; improper potential control can lead to loss of corrosion protection; and some issues with cathode fouling and erosion have been reported. Galvanic Coupled Anodic Protection (GCAP) systems have not been widely utilized in industry, but offer some solutions to these issues. GCAP systems have been developed using Pt and Au as the cathode materials for use in H2SO4. Previous work on the oscillatory behaviour of austenitic stainless steels indicates that nickel sulphide (NiS) could be used as cathode material in a GCAP system in concentrated H₂SO₄ to protect stainless steel. The objective of this study was to develop a better understanding of the behaviour of NiS when galvanically coupled to Type 430 stainless steel to determine whether it can be used in a GCAP system. NiS and NiS(Ni) electrodes are able to provide the Icrit needed to passivate Type 430 at anode/cathode ratios of 10:1, while NiS(Ni) electrodes were able to provide the Icrit needed to passivate the Type 430 stainless steel at a ratio of 20:1. In addition it was shown that the NiS(Ni) electrode was able to maintain passivity of the Type 430 stainless steel array using an anode/cathode surface area ratio of 100:1. NiS was shown not to be inert in concentrated H₂SO4 and corrosion rate calculated via Tafel extrapolation and shown to be 0.014 mm/yr at room temperature and 0.128 mm/yr at 60 ºC.</p>