Uncoated compressor components are more susceptible to corrosion, erosion, and fouling. Corrosion and erosion in compressors can lead to the premature replacement of components due to pitting and material loss. Fouling can reduce efficiency as it builds up on the surface of the airfoils.
Although there are various forms of fouling, both corrosion fouling and particulate fouling are the most pertinent in turbomachinery.
- Corrosion fouling is a chemical reaction between the liquid in the gas and the airfoil. Many metals form an adherent oxide layer to passivate the surface and prevent further corrosion. The oxide layer typically exhibits high frictional properties.
- Particulate fouling is a buildup of small particles ingested by the compressor. These particles build up a kind of a sand, distorting air flow and reducing the compressor’s capacity and efficiency.
Sulzer offers two types of compressor coatings:
- The first type, including HICoat A08 and A21, is designed to improve surface finish and corrosion resistance.
- The second type, designated A24, is also corrosion resistant. But it is specifically designed to reduce particulate fouling by providing release characteristics.
HICoat A08 and A21 are metallic-ceramic coatings useful in most situations where compressors must resist corrosion or light erosion, and can benefit from a fine surface finish. Metallic-ceramic coatings are used most predominantly today due to their:
- Corrosion protection capabilities
- Versatility of use on large components
- Ability to restore surface finish
- Low cost of application
Stationary and rotating compressor blading, diaphragms, guide vanes, and shrouds are just a few of the countless components which greatly benefit from this coating technique.
HICoat A24 is a metallic-ceramic-polymeric coating. The organic topcoat of this coating is impregnated with PTFE (Polytetrafluoroethylene) to provide release characteristics and improve antifouling protection.
Compressor coatings A08, A21 and A24 resist corrosion by providing sacrificial cathodic protection. This technique uses preferential corrosion of an active coating layer in order to protect a less-active metal underneath. Average coating thickness is 25–75 microns (1–3 mils) for A08 and 50–100 microns (2–4 mils) for A21. Roughness typically ranges between 10 to 30 Ra (μin) at 0.01 in. cutoff. This system provides significant improvements in aerodynamic efficiency over bare martensitic stainless steels throughout the life of the blade.