ELECTROCHEMICAL CORROSION TESTING OF HIGH PHOSPHORUS ELECTROLESS NICKEL IN 5-PERCENT NACL

Electroless nickel coatings are increasingly used in a variety of applications due to their advantageous chemical, mechanical, and physical properties. The purpose of this research is to investigate the mechanism of corrosion protection of high phosphorus (10 to 11 wt% P) electroless nickel coatings in 5% NaCl using electrochemical techniques. The results indicate a very low corrosion rate (less than 1 mpy [25.4-mu-m/y]) in this environment at the open-circuit corrosion potential. The corrosion rate reached a steady-state after approximately one day, indicating accelerated corrosion testing and coating evaluation can be achieved using electrochemical techniques. Controlled hydrodynamic tests using a rotating cylinder electrode indicate the corrosion protection film is sensitive to wall shear stresses induced by fluid flow. At the open-circuit corrosion potential, it is proposed that the alloy is protected by suppression of the cathodic reaction kinetics, which may be the result of the formation of a phosphorus-enriched film on the alloy surface.