In an ideal world, a material that is inherently resistant to its service environment complies with the mechanical, formability and economic criteria would be the first preference selection. Sadly, this is not reality. Many materials require a method of corrosion control.
There are three main approaches:
- Modification of the environment to which the material is exposed
- Electrical methods of control
- The use of protective coating
Thermal spray coatings are commonly used in preventing corrosion of many materials. Often, additional benefits are achieved such as improved wear resistance. This is due to the wide variety of coatings that can be applied.
There are three main methods using thermal sprayed coatings as corrosion protection:
- Neutral barrier coating
- Cathodic barrier coating
- Sacrificial anodic coating
Neutral Barrier Coating
Neutral materials such as alumina or chromium oxide ceramics provide excellent corrosion resistance to most corrosive environments by the exclusion of the environment from the substrate. Generally, a neutral material will not accelerate the corrosion of the substrate even if the coating is somewhat porous, although any corrosion of the substrate interface with the coating should be avoided to prevent coating separation. Sealing or impregnation of the thermal sprayed coating is recommended. Thicker and less porous layers have better barrier properties to the substrate and therefore exhibit superior performance.
Cathodic Barrier Coating
Cathodic Barrier Coatings are coatings which are cathodic with respect to the substrate material. A stainless steel or nickel alloy coating would be cathodic to a steel substrate. Cathodic coatings can provide excellent corrosion protection. However, there are limitations; there must be a complete barrier to the substrate from the environment. If the substrate is exposed to the corrosive environment, the substrate will become the anode and corrosion will be dramatically accelerated resulting in spalling of the coating. Sealing or impregnation the coating is recommended. Thicker and less porous layers have better barrier properties to the substrate and therefore exhibit superior performance.
Sacrificial Anodic Coatings
Anodic coatings for the protection of iron and steel substrates are almost totally limited to zinc and aluminium coatings or their alloys. Where coatings anodic to the substrate are applied, the corrosion protection is referred to as cathodic protection or sacrificial protection. The substrate is made to be the cathode and the coating the sacrificial corroding anode. An ordinary thermal sprayed coating of zinc or aluminium although somewhat porous, to a large extent, excludes the environment and provides cathodic protection. Where desired the porosity can be sealed with organic sealers, or the coating painted, which can in some cases prolongs the life of the protective system by increasing the barrier effect. This method is generally regarded as providing superior corrosion protection versus galvanizing, plating and painting without excessive cost penalties.