Corrosion and Wear Behavior of Electroless Nickel Coatings

Introduction

Corrosion is an aspect of the decay of materials by chemical or biological agents. Deterioration of materials by physical causes is not corrosion and is described as erosion or wear. Thus corrosion may be described as the surface wastage of metals that occurs when metals are exposed to reactive environments or in another way it is the destructive attack of a metal by chemical or electrochemical reaction with the environment. Wear in simple terms can be defined as the erosion of material from a solid surface by the action of another material which comes in contact with it. In most practical situations however, chemical attack accompanies physical deterioration and the combined result is termed as corrosive wear.

Corrosion and wear are two deteriorating phenomena which affect various devices and their service lives. Corrosion and wear cause environmental contamination, personal injuries and untimely shutdown of various machineries. Hence, no established industry can tolerate these deteriorating phenomena. Corrosion is frequently encountered in our daily life. The pipes which are used for water supply get corroded due to continuous flow of water. The iron rods placed inside the reinforced concrete of a building can also get corroded. The metal structure of bridge over the river is very much prone to corrosion. Marine sector is one of the worst affected by corrosion. Components of boats and ships sailing over the seas, onshore and offshore installations, etc. are all highly prone to corrosion due to their salt laden environment which accelerates the corrosion process. In power plants, the blades and bearings of the steam turbine, components of water pump are mostly affected due to corrosion. Again, wear occurs wherever two bodies come in contact and there is relative motion in between them. Wear is also encountered over a large practical domain ranging from the sole of a shoe to different sliding components in machinery. Wear is commonly associated with friction which results in huge energy losses in industries.

Corrosion and wear may lead to failure of infrastructures like, electrical and plumbing lines, chemical processing plants, power generation plants, and aircraft and marine equipment which are usually costly to repair. However undesired they might be, corrosion and wear cannot totally be eliminated; instead be controlled in a sustainable manner. The main motive behind controlling corrosion and wear are conservation, protection, economics and safety. The methods by which the surface of a metal can be protected against corrosion and wear are fall under the realms of surface engineering. Surface engineering refers to technologies that aim to design and modify the surface properties of components. Coatings are a well-established form of surface engineering. Several types of coating processes exist which provide protection of the metal surface against corrosion and wear. Among these, electroless nickel (EN) has emerged as a protective coating and has found widespread industrial acceptance due to its valuable properties (Sahoo and Das, 2011). Electroless nickel coatings are deposited chemically and well known for their deposit uniformity. These coatings are smooth and can be deposited virtually on any type of substrate. Moreover, the properties of electroless nickel coatings can be customised to suit a particular application by the incorporation of additional compatible elements.

Figure 1.

Electroless coating setup