Corrosion is an unavoidable problem that occurs almost everywhere. It can happen when a solid interacts with its environment. Corrosion oxidises metals back to some natural ore. Despite not all corrosion is unwanted, corrosion has become a growing economic concern. Taking an example of what happens in USA, corrosion of metals costs the economy almost $300 billion per year at current prices, which translates to 3-4% of GDP. Besides the economic concern, corrosion may adversely affect the environment. Worse than all those mentioned, corrosion can cause unexpected death from corrosion-related accidents. For example, the sudden collapse caused by corrosion fatigue of the Silver Bridge over the Ohio River in 1967 cost 46 lives on top of the needless-to-say millions of dollars material loss. Therefore, the benefits brought about by corrosion are outweighed by its detriments. The fact is, about 33% of these costs could be diminished by using corrosion-resistant materials.
Being defined as the sideways displacement of material from its original position on a solid surface performed by the action of another surface, wear is another mechanical property of materials that materials scientists consider when selecting materials in a design task. In order to be wear-resistant, a material must be hard and strong.
The demand for corrosion- and wear- resistant coatings is nowadays rapidly increasing, especially in oil and gas industries. The most common solutions are Hard Cr and Quench-Polish-Quench (QPQ) processes. However, these two processes affect the environment adversely. As alternatives, coating layers can be introduced to metal samples. Coatings can be done with or without electricity. If electricity is involved, the process is called electroplating. It is done by immersing the sample that is going to be coated together with an anode of the coating metal in an electrolyte containing salts of the metal to be plated. The other coating method that relies on chemical reaction instead of the surge of current is termed electroless plating.
One popular electroless plating is the electroless nickel (EN) plating process. This process is first invented in 1940s by Brennel and Riddell. The coating will be deposited on a metal/ alloy surface submerged in an aqueous solution containing nickel ions, complexing and buffering agents, a reducing agent, and stabilizers. One advantage of using electroless plating is that a uniform thickness can be obtained. Also, the longer the metal is submerged, the thicker is the coating deposition. EN plating does not only improve corrosion- and wear-resistance of a material, but also its hardness, lubricity, solderability, bondability, uniformity of deposit regardless of geometries and nonmagnetic properties of high-phosporous nickel alloy. As such, the demand for electroless plating has burgeoned over the past decades, even though it is considered a relatively new process and the relatively more expensive cost than electroplating. It has a really broad range of applications too – to name a few: automotive, aircraft, chemical equipment, railroad, electrical motors, printing rolls, oil wells and mining, molds, military, valves, textile and electronics.
The chemical reactions that occur when using sodium hypophosphite as the reducing agent in electroless nickel plating are portrayed by the four equations below: 1. H2PO2- + H2O HPO32- + 2H + H+ 2. Ni2+ + 2H Ni + 2H+ 3. 2H H2 4. H2PO2- + H P + OH- + H2O
Sometimes, materials are coated not only with EN solution, but also with EN composite (i.e. EN solution mixed with dispersant and ceramic composite powder such as TiB2, TiC or ZrB2). These composite powders increases hardness, roughness of the materials and increase their corrosion- and wear-resistance.