When Stainless Isn’t So Stainless

Excerpted from MPI’s Coatings Specialist training and the experience of Paint Quality Assurance Inspector Dave Lick

“Stainless“ steel is the name given to a group of corrosion resistant and high temperature steels that possess in excess of 10.5 - 11% chromium content. Their remarkable resistance to corrosion comes from the chromium-rich oxide film that forms on the surface. In the presence of moisture, ordinary carbon steel forms iron oxide (rust); rust is not protective and eventually the entire piece of steel will corrode and be converted to rust. But when enough chromium (more than about 10%) is added to ordinary steel, the oxide produced on the surface is a very thin, virtually invisible layer that provides protection from a wide range of corrosive media. This is “stainless” steel and there are different types and grades that vary widely in corrosion resistance, strength, and performance properties; common types include austenitic, ferritic, martensitic, and duplex.

Why paint stainless steel?

Stainless steel is often used without a coating except for esthetic purposes (e.g. if the architect wants to change the color). However, it is important to note that all stainless steels are not created equal: there are various grades of stainless, and cheaper grades will corrode when exposed to marine environments, chemicals, or high humidity. Such grades may require a rust-inhibitive primer or system for protection just as mild steel would.

That’s what the intrepid inspector found when called to investigate a problem at an upscale handsome new pool complex...

The Case of the Not-So-Stainless Indoor Pool Duct

The concrete ceiling above the expansive indoor pool was coated with a white epoxy, and carried a large (5+ feet in diameter) stainless steel air duct. Six months after the facility opened, telltale spots of red and white rust corrosion product started forming on the duct’s joints and connections—and started to spread. When bits of rust started falling into the pool, the exasperated facility owner took action. His frustration was compounded by the fact that the stainless steel hand railings around and inside the pool were in flawless condition, even though their exposure environment of immersion, splash and spill was more aggressive than what the rusting duct above was exposed to.

The inspector’s subsequent investigation determined that the duct was made of low-grade stainless steel brought in from overseas (proving once again the age-old adage: you get what you pay for) —while the hand railings, which came from a different source, were indeed high-grade stainless steel appropriate for marine exposure.

Installing a new duct was out of the question, so the only solution was to paint the rusting duct with an anti-corrosive paint system. However, the use of conventional methods for surface preparation and paint application were out of the question: the new state-of-the-art pool with its fine tile walls and floors and adjacent exercise area were the ground floor of an occupied high rise condominium, and the owner demanded the work be done without generating excessive noise, dust or debris, let alone overspray.

Surface Prep for Stainless Steel

Stainless steel surface preparation requires two steps: remove any surface contaminants present (such as dirt, oil, grease, and other foreign matter), then roughen (profile) the surface. Since stainless is a smooth substrate, it can be difficult to obtain adequate adhesion for the coating system, so the preferred method for roughening the surface is a sweep blast (SSPC-SP 7 Brush-Off Blast Cleaning).

Since neither blasting nor any form of power tools could be used on this project, the inspector recommended application of a highly concentrated solution of MPI #25 Cleaner, Etcher for Galvanized Metal to clean and dull the surface — and since spraying was not an option, the solution was sponged and wiped on to avoid dripping on to any adjacent surfaces. After carefully rinsing the surface, the ducts were then hand-sanded with 150-grit silicon carbide abrasive paper, in an effort to assure some kind of tooth on the surface.

A test patch was then coated with the new paint system: an epoxy primer (MPI #101) topcoated with a 2-part aliphatic polyurethane (MPI #72). Much to the inspector and owner’s relief, the adhesion of the test patch proved sufficient, so the rest of the duct was brushed and rolled with the system — and one year later, the duct is still in pristine condition.

The net cost? $25,000 to prepare and coat the duct, and for the owner, his pool out of service for one week, plus the aesthetic loss: his gleaming new natatorium now has a white duct on a white ceiling, instead of sparkling stainless steel. So here’s a lesson to the specifier, owner, or GC: if a building product is classified as “stainless steel” and especially if the price appears to be a bargain, ask the manufacturer exactly what type and grade of stainless steel they’re selling, and verify if it is suitable for its intended use. If not, your “Stainless” may turn out to be “Not So Stainless.”

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