Electroless nickel for corrosion control in chemical, oil and gas industries

The electroless nickel (EN) coatings (a group of coatings) have truly unique engineering properties - uniformity in thickness, high corrosion resistance, high hardness (500 to 900VHN), good wear resistance, inherent lubricity, easy solderability and brazeability, adjustable electric and magnetic properties. These unique properties are due to the nanocrystalline nature of the deposits, e.g. in EN nickel-phosphorous deposits grain size range between 10-12 A degrees to 150 A degrees (1-1.2 to 15 nm) depending on the phosphorous content in the deposit. Corrosion resistance has been one of the primary attributes of EN coatings. EN is a barrier coating, protecting the substrate by sealing it off from the environment. The corrosion resistance is further enhanced by the formation of glassy passive oxide film on the coating surface. The higher phosphorous (10 to 12 % P) EN coatings are more resistant to acidic corrodents and products that hydrolyse to form acids. Lower phosphorous (1 to 4 % P) EN coatings are very resistant to strong alkalis. EN can withstand the corrosive environments encountered in oil and gas wells and more importantly the "holidays" (pores and scratches) are not the sites for accelerated corrosion even when sulphide is present in the crude oil. EN coatings are being successfully used for corrosion protection of all the 'boiler works' - heat exchangers, compressors, valves, ducts etc. against corrosive fluoride vapours by the French Atomic Energy Commission. EN coated spheroidal graphite iron and lead connectors and fittings are being commercially used for combating corrosion, the former in the natural and LP gas industry and later, as cost effective replacement for DZR brasses which were sometimes prone to dezincification and bacterial corrosion. EN coatings exhibit resistance to S.C.C., a phenomenon that had caused catastrophic failure of steel in high temperature caustic and chlorine service. The successful applications of EN for corrosion protection depends on the proper surface pre-treatment, high performance and properly controlled operation of the selected EN plating bath and the conformance to specifications/standards. ALL these factors are described in detail for maximising the corrosion resistance performance of EN coatings. A number of successful cases on the use of EN coatings in the industry are described in the paper.