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Painting Options for Corrosion Protection of Gearboxes

By: Chris Popp, DieQua Corporation

Rust never sleeps.

I doubt that Canadian rocker, Neil Young, had gearbox corrosion protection in mind when he put out his iconic album in 1979, but it’s certainly a relevant concern in many industrial environments. Cast iron, steel and aluminum may all need some level of protection applied to ward off the ravages of corrosion.

The harshness of the environment, its chemical makeup, or its ambient condition are some of the main determinants of what type of protection is needed to avoid corrosion of gearbox housings. It’s not as easy as just slapping a coat of paint on it.

Or, in some cases, maybe it is. In most factory situations, protection against general humidity is the primary concern. Unfortunately not every machine is operating in the most ideal conditions. Therefore, different levels or kinds of protection are necessary in different situations.

The international standard ISO 12944 classifies corrosion environments into 6 basic categories, with typical examples:

C1 – Heated buildings, neutral atmosphere
C2 – Rural areas, low pollution, better protection
C3 – Industrial atmospheres, high humidity production areas
C4 – Chemical processing, acidic or alkaline gases
C5I – High humidity and aggressive atmospheres
C5M – Marine, offshore, coastal areas with high humidity

DieQua offers painting options for each of these environments. Standard procedures include using either polyurethane or epoxy coatings in a variety of layers. The sequence of layering for the various classifications include:

C1 – Either a standard primer or a dip primer then 2 part polyurethane surface layer. This allows a base for the customer to apply his own paint.
C2 – A dip primer, applied primer and either a surface layer of either 2 part polyurethane or 2 part epoxy.
C3 - A dip primer, applied primer, and intermediate primer, then a 2 part epoxy surface layer.
C4 - A dip primer, applied primer, 2 layers of intermediate primer, then a 2 part epoxy surface layer.
C5I and CFM - A dip primer, standard primer, 2 layers of intermediate primer, then a 2 part polyurethane surface layer.

What are the differences between epoxy and polyurethane? The basics are that Epoxy has excellent resistance to chemicals, acids and bases, but is not resistant to ultraviolet light. While not as resistant to harsher acids and bases as epoxy, polyurethane is resistant to UV light so therefore maintains its color better over time.

For food processing applications, a USDA approved white multi part epoxy coating is a popular option that we often provide. Another possibility for these applications is a product using a unique stainless steel leafing pigment. The most well known formula is marketed under the Steel It brand name. This we offer as well.

All paints and coatings wear over time. Therefore, knowing the desired lifetime can aid in selecting the proper protection. It should be noted that, regardless the level of protection, corrosion resistance can be compromised if the surfaces are nicked or chipped. For this reason, it pays to handle painted and coated gearboxes with some care during transport and installation.

Because of some of the limitations of paint, and the care required, stainless steel housing material is growing in demand. That growth, however, is being tempered by its high cost. Bt properly selecting the appropriate paint protection level and taking care during gearbox installation, many years of cost effective service can be enjoyed without the need for “going stainless”.

07/2014

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