Painting stainless steel and aluminum: why and how?

You paint stainless steel and aluminum when their passive layer is no longer enough: chlorides, stainless steel under hot insulation, acidic or alkaline environments, or contact with another metal. Preparation means degreasing, then a light blast with non-ferrous abrasive that creates a shallow profile without contaminating the surface. The primer is chosen to bond to these metals, and connections between dissimilar metals are isolated to prevent galvanic corrosion.

Stainless steel and aluminum protect themselves with a very thin oxide layer. That layer reforms on its own in a favourable environment, but it can break down locally, and the corrosion that follows is often fast and concentrated. Painting these metals then calls for different preparation than carbon steel, because paint bonds poorly to a smooth, passive surface. This guide is written for maintenance managers and project managers in Quebec and Ontario.

Why paint a metal that does not rust?

Because "stainless" does not mean immune: in some environments the passive layer breaks down and corrosion becomes localized. Paint then forms a barrier against the aggressive agent, or isolates the metal from another metal.

• Chlorides. Sea spray, de-icing salts and chlorinated cleaners cause pitting on stainless; see our article on pitting corrosion.
• Stainless under hot insulation. Water that gets under insulation concentrates chlorides on a hot surface, with a risk of pitting and stress corrosion cracking. NACE SP0198 recommends coating these surfaces; see corrosion under insulation.
• Aluminum in alkaline or acidic environments. Contact with fresh concrete, caustic washes or acidic condensate attacks the oxide layer.
• Contact between metals. Aluminum connected to stainless or carbon steel corrodes faster when moisture is present.
• Identification and appearance. Safety colours, piping codes or a uniform look across an installation.

How do you prepare stainless steel and aluminum for painting?

You first degrease with solvent to SSPC-SP 1, then create a light, uniform profile by brush-off blasting with non-ferrous abrasive, per SSPC-SP 16, which specifically covers galvanized steel, stainless steel and non-ferrous metals. The goal is not to remove metal, but to make the surface rough and clean for adhesion.

Three rules set this preparation apart from carbon steel work:

• Non-ferrous abrasive. Garnet, aluminum oxide or another clean non-metallic abrasive, never steel grit. Iron particles embedded in stainless rust and create stains and pit initiation sites. Our guide to choosing a blasting abrasive compares the options.
• Dedicated tools. Brushes, discs and tools used on carbon steel never touch stainless, for the same reason.
• Light profile. Aluminum is soft: overly aggressive blasting distorts it, especially thin sheet, and creates too deep a profile. Pressure, grit size and stand-off are set on a trial.

Before blasting, all oil, grease, marker ink and adhesive residue must be gone, because blasting spreads them instead of removing them. After blasting, primer goes on quickly: aluminum and stainless reoxidize too, and a fresh oxide layer reduces adhesion. A trial patch, prepared and tested for adhesion before production, confirms that the preparation and primer work together on the actual metal.

For fragile parts or sensitive surfaces, our glass bead and dry ice blasting service offers gentler methods. On aluminum, some systems allow or require a chemical pretreatment per the data sheet.

Which primers suit these metals?

You choose a primer formulated and documented to bond to stainless or aluminum, usually an epoxy family, and check its compatibility with the service temperature and the topcoat. The data sheet should explicitly name the metal as an accepted substrate.

Two watch points come up often. Zinc-rich primers add nothing on stainless or aluminum and should be avoided on these substrates. For stainless, specify a product low in chlorides and halogens, so the coating itself does not bring the agent that attacks the metal.

How do you prevent galvanic corrosion at connections?

You electrically isolate dissimilar metals, keep water from standing at the contact and, if you paint, paint the more noble metal or both, never the less noble metal alone. Aluminum with stainless, or aluminum with carbon steel, is the most frequent pair in plants.

• Isolation. Non-conductive washers, sleeves and gaskets between parts and around bolts.
• Sealing. Compatible sealant in crevices to prevent a permanent electrolyte.
• Paint the noble metal. If only the less noble metal is painted, every film defect concentrates corrosion on a small area facing a large cathode.
• Fasteners. Chosen to be compatible with the joined parts; see our article on bolt and connection corrosion protection.

Our galvanic series table and our article on common galvanic couples help assess the risk of a connection.

Should stainless be passivated before painting?

Not always. Passivation removes free iron and restores the passive layer, which is useful after welding, grinding or contamination; it is done to ASTM A380 or ASTM A967. If the stainless is to be painted, the sequence (passivation, light preparation, primer) is confirmed against the data sheet.

Our article on stainless steel passivation covers the methods. The same light-preparation principle on a non-ferrous metal applies to galvanized steel; see our article on duplex systems on galvanized steel.

What checks follow painting?

You check dry film thickness, adhesion and appearance, as for steel, but adhesion deserves more attention because it is the weak point on these substrates. Results are recorded with the preparation carried out.

• Thickness. Gauge suited to non-ferrous metal, per SSPC-PA 2.
• Adhesion. Pull-off test to ASTM D4541 or tape test to ASTM D3359, on the structure or on panels prepared the same way.
• Appearance. No rust staining on stainless, a sign of iron contamination.

When is it better not to paint?

When the metal sits in an environment where its passive layer holds well and no galvanic contact is a concern, paint adds maintenance without benefit. Clean, passivated stainless in a dry indoor atmosphere, for example, often needs no paint. The decision is made at diagnosis, as part of our protective coatings work.

How do you get a quote?

Send photos, the stainless grade or aluminum alloy if known, the service environment, the temperature and the connections involved. Our crews and qualified subcontractors then propose the right preparation and system.

Our industrial painting page presents our systems. We work from LaSalle, including in Hamilton and across Ontario and Quebec. Submit a request for a quote: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Can stainless be blasted with the same grit as steel?

No. Steel grit leaves embedded iron particles that rust on stainless. Use a clean non-ferrous abrasive reserved for these metals.

Does paint stick to smooth aluminum?

Poorly. You must degrease, then create a light profile or apply the pretreatment set by the data sheet, before a primer formulated for aluminum.

Should stainless under insulation be painted?

Often yes, especially if it is hot and exposed to chlorides. NACE SP0198 covers these coatings, and the product must withstand the service temperature.

How do you know whether an aluminum and stainless connection will corrode?

Look at the gap on the galvanic series, the area ratio and the presence of moisture. A frequently wet connection with a small aluminum part is the most exposed.