Salt Spray Testing of Sacrificial and Barrier Type Coatings for the Purpose ofFinding a Corrosion Resistant and Environmentally Acceptable Replacement for Cadmium Plate
Authors: Schultz Eric J. , Haeberle Timothy 
Published: October 2008

Cadmium plate is used to protect various components of offshore oil and gas production equipment from surface marine environments such as salt spray. As a result of concerns about the toxicity of cadmium metal and it's processing solutions, cadmium plating is coming under increasingly restrictive and prohibitive American and European governmental legislation. This research project was performed to find an environmentally acceptable coating which provides equivalent or superior resistance to surface marine corrosion when compared to cadmium plate. In order to find a replacement for cadmium plate, a large number of sacrificial and barrier type coatings were exposed to an accelerated salt spray test in accordance with ASTM B117-94. A sacrificial coating is defined as a coating that exhibits a higher potential to corrode in salt spray than the underlying substrate thus preventing corrosion of the substrate. A barrier coating is defined as a coating which forms an impermeable barrier between the coating and the substrate effectively isolating the substrate from the corrosive environment. The only sacrificial coating which resisted 1000 hours of accelerated salt spray testing without any indication of failure was the 0,0006? thick zinc-nickel plate with an olive drab chromate treatment. Based on these test results, zinc-nickel plate is recommended as a corrosion resistant and environmentally acceptable replacement for cadmium plate for use in surface marine environments. Electroless nickel coatings with a minimum applied thickness of 0.002 also resisted 1000 hours of accelerated salt spray testing without indication of failure. Electroless nickel is not recommended for corrosion resistance in salt spray environments for two reasons. Electroless nickel is susceptible to microcracking when heat treated at moderate to high temperatures. Heat treatment improves the hardness and resultant wear resistance of the coating. Microcracking will compromise the integrity of the coating resulting in pitting, cracking or crevice corrosion of the substrate in corrosive environments. Secondly, any significant mechanical damage to the coating or disbanding of the coating substrate interface will also result in corrosive attack of the substrate.

Introduction

Cadmium plate is commonly used to protect carbon and lowalloy steels from a variety of corrosive environments. Three common environments in which cadmium is used are surface marine exposure or salt spray (e.g., offshore oil and gas production equipment), land transportation which involves exposure to combustion engine exhaust fumes, road salts, etc. (eg., vehicle equipment) and industrial, which includes exposure to chemicals, gases and other hazards (e.g. refinery, chemical, power plant and factory equipment).

The cadmium plating process uses cadmium metal and cyanide bearing solutions which are toxic to human health and the environment. As a result, the cadmium plating process is coming under increasingly restrictive and prohibitive environmental regulations.