MYTH: MACHINERY RUST AVOIDANCE — IT’S JUST A BUCKET OF PAINT AWAY

Irecently accompanied my wife to a nail salon and watched in utter fascination the level of care and sophistication with which the young manicurists treated and painted my wife’s nails. There were several cleaning steps, surface pre-treatment and, as far as I can remember, at least three layers of coatings and colored nail paints. This I felt was all a little exaggerated but I was smart enough not to voice this opinion to my wife.

On the other hand, I often visit gas turbine and compressor installations and, especially in older facilities, what I observe is that the paint on the machines is in poor condition, rust spots are everywhere. So why can’t we put the same love and care into painting gas turbines than my wife puts into painting her nails? A multi-million dollar gas turbine certainly deserves to be treated with care and attention, albeit maybe not as much attention as my wife requires.

Gas turbines and compressors are often exposed to harsh environments, such as saltladen offshore, contaminated industrial or sandy desert environments. In these environments, an appropriate paint and coating system that protects the machine’s metal parts from rapid corrosion is imperative.

Most turbomachinery manufacturers use advanced paint systems that have been specifically designed for the type of metal, surface roughness, metal temperatures, operating temperature cycles, level and type of environmental exposure, and required mechanical hardness to assure maximum machine protection.

The process usually consists of surface cleaning, pretreatments, and multiple layers of epoxy, polyurethane, polysiloxane or powder coatings. There are usually rigorous quality standards for the coatings including appearance, coverage, surface finish (acceptable surface roughness), thickness, adhesion, elongation, corrosion resistance, heat resistance, and so on, that must be maintained.

Painting quality inspections are typically performed on both coupons (samples) and on the actual parts. Thus, a proper paint system consists of different types of paints, application methods, and quality control individually specified for each component of a machine.

For example, whereas single-layer epoxy is usually adequate for interior components that are not continuously exposed to the elements, a three-layer coating (i.e., inorganic zinc, epoxy and polyurethane) is often used for external and environmentally exposed surfaces. Also, hot or oiled surfaces sometimes require special coatings to assure longevity.

In almost all cases when a machine leaves the factory, the originally applied painting is acceptable and will provide several years of excellent surface protection. But over time, all paints degrade because of harsh wind, sun, rain exposure, metal expansion and contraction from thermal cycling, machinery vibrations, and simple mechanical surface abrasion or impacts.

Surface wear, loss of adhesion, flacking, and spallation of the original paint will eventually occur, exposing the bare metal to the environment. Repainting and touch-up is usually performed by the machinery operators in the field.

Unfortunately, this repainting seldom follows the same rigorous quality standards as the original factory paint system. Consequently, after a short time, the paint severely degrades, the metal surfaces are again exposed to the environment, and operators are often unwilling to invest in additional downtime and cost for another inadequate paint job.

Most metals corrode rapidly if not protected and over time rust spots become a common sight in the facility. Operators need to maintain the same high-quality paint system standards for painting and repainting of equipment that they would apply to other maintenance items of the machine.

Because of poor experience with paint quality, some operators insist on using stainless steel instead of carbon steel for low-temperature gas turbine components, such as inlet ducting, enclosure housing, and common low temperature pipes and fittings. Similarly, API 616 5th edition introduced an across-the-board requirement for stainless steel of the entire gas turbine air duct and filter house.

Although this brute-force approach avoids corrosion problems, stainless steel is many times more expensive than painted carbon steel and, unless the machine is installed in an extremely severe marine environment, stainless steel is usually not worth the high-cost differential for the limited added protection benefit. A properly designed and correctly applied paint system will provide adequate corrosion protection for most low-temperature machinery components in almost all environments.

All machinery manufacturers provide their customers with detailed paint specifications. If these are properly followed when repainting in the field, the machine should be adequately protected for several years. The additional cost for implementing a high-quality paint system for repainting of older machines is well worth it when compared to the loss of value of a severely corroded machine or the high cost of stainless steel components.

Authors

Klaus Brun is the Machinery Program Director at Southwest Research Institute in San Antonio, Texas. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.

Rainer Kurz is the Manager for Systems Analysis at Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and the chair of the IGTI Oil and Gas Applications Committee.