Coating Stainless Steel
When and why stainless steel is coated — corrosion under insulation, chlorides, galvanic isolation and appearance — and how to prepare its passive surface for good adhesion.
Key takeaways
- Stainless steel resists general corrosion through a thin chromium-rich passive film, but it can still suffer pitting, crevice corrosion and chloride stress corrosion cracking.
- The most common reasons to coat stainless are corrosion under insulation, chloride-rich or splash environments, galvanic isolation, and appearance.
- Its smooth, passive surface gives poor adhesion unless it is cleaned and roughened, typically by sweep blasting with clean, non-metallic abrasive.
- Avoid contaminating stainless with carbon steel particles and avoid zinc-containing coatings where the steel may see high temperatures.
Stainless steel is usually chosen precisely so that it does not need painting. Yet many plants coat stainless steel equipment, piping and structures — sometimes for good engineering reasons, sometimes for appearance. When coating is needed, the stainless surface behaves very differently from carbon steel, and the preparation and product choices must reflect that.
This article covers why and when stainless steel is coated, how to prepare it, and which coating types are commonly used.
Why coat stainless steel?
Stainless steels contain at least about 10.5% chromium, which forms a self-healing passive oxide film. That film can break down locally, especially in the presence of chlorides, elevated temperatures and stagnant conditions. Common reasons to coat include:
- Corrosion under insulation (CUI). Wet, chloride-bearing insulation can cause pitting and stress corrosion cracking of austenitic stainless steels in an intermediate temperature range — commonly cited as roughly 50–175 °C (120–350 °F). NACE SP0198 addresses control of corrosion under insulation and recommends coatings for susceptible equipment.
- Pitting and crevice corrosion in marine, coastal or chemical environments. See pitting and crevice corrosion.
- Galvanic isolation where stainless contacts carbon steel, aluminum or galvanized components; see galvanic corrosion.
- Tea staining and aesthetics on architectural stainless in coastal or polluted air.
- Color coding and safety marking.
Grades and what they mean for coating
| Family | Examples | Main coating concern |
|---|---|---|
| Austenitic | 304, 316 and their low-carbon variants | Chloride stress corrosion cracking under insulation; pitting in chlorides |
| Ferritic | 430 and similar | Lower pitting resistance; tea staining outdoors |
| Duplex | 2205 and similar | Better chloride resistance, but still coated under insulation in some practices |
| Martensitic | 410, 420 and similar | Lower corrosion resistance; more often coated for service |
Owner and industry practices differ on which grades and temperature ranges require coating, so always follow the project specification.
Surface preparation
The passive film and smooth mill finish make stainless steel a poor surface for adhesion. Preparation must roughen the surface without contaminating it.
- Degrease. Remove oils, markings and fabrication residues with a chloride-free cleaner and rinse with clean, low-chloride water.
- Protect from contamination. Use stainless-only brushes and tools, and never use abrasive previously used on carbon steel.
- Sweep blast. Roughen with clean, non-metallic abrasive such as aluminum oxide or garnet to create a uniform angular profile, often in the range of about 1.5–3 mils (40–75 µm) or as the coating requires. SSPC-SP 16 covers brush-off blast cleaning of non-ferrous metals and is commonly applied to stainless.
- Remove dust. Blow down with clean, dry air and vacuum.
- Prime promptly. Apply the primer before the surface is contaminated or the passive film fully re-forms.
Guidance on choosing abrasives appears in abrasive media selection.
Embedded iron particles from carbon steel abrasive, grinding discs or wire brushes rust quickly and can initiate pitting on stainless steel. Keep stainless tooling and abrasives segregated.
Common coating options
Epoxy and epoxy phenolic
High-build epoxies and epoxy novolacs are widely used on stainless steel at ambient and moderate temperatures. Epoxy phenolic and novolac systems extend service into higher temperature ranges and are common for CUI protection; see phenolic and epoxy-phenolic linings.
High-temperature and cyclic-service coatings
Inert multipolymer matrix and silicone-based coatings are used where equipment cycles through wide temperature ranges or operates hot.
Thermal spray aluminum
Thermal spray aluminum (TSA) is used on stainless and carbon steel under insulation for long-term protection, often with a sealer. See thermal spray metallizing.
Architectural finishes
Polyurethane, fluoropolymer and powder coatings are used where color is needed, applied over a primer designed for non-ferrous metals.
Zinc-rich primers are generally avoided on austenitic stainless steel that may be exposed to high temperatures, welding or fire, because molten zinc can cause liquid metal embrittlement of the steel.
Design and specification tips
- Specify low-chloride materials. Coatings, thinners and cleaning products used on austenitic stainless should not introduce chlorides.
- Match temperature ratings. Under insulation, the coating must withstand both the maximum operating temperature and thermal cycling.
- Isolate dissimilar metals. Coat the more noble metal (often the stainless) or use insulating gaskets and sleeves to reduce galvanic attack on the other metal.
- Verify adhesion. Use pull-off or cross-cut testing on trial areas to confirm preparation; see adhesion testing.
- Follow the data sheet. Confirm that the product is approved for stainless substrates and note any required primer.
When not to coat
Coating stainless steel in mild environments may add cost without benefit, and a damaged coating can create crevices that encourage localized corrosion. In many cases, correct grade selection, good drainage, smooth finishes and regular washing are better solutions than painting. Coating is most justified where the risk is clear — CUI, chlorides, galvanic couples — or where appearance is a requirement.
Frequently asked questions
Will paint stick to stainless steel?
Not well without preparation. Clean, roughen with non-metallic abrasive and use a primer approved for stainless or non-ferrous metals.
Why coat stainless steel under insulation?
Wet insulation can concentrate chlorides on hot stainless surfaces, causing pitting and stress corrosion cracking. A suitable coating separates the steel from that environment.
Can I use steel grit on stainless steel?
Avoid it. Iron contamination from steel abrasive can rust and initiate pitting. Use clean non-metallic abrasives dedicated to stainless work.
Is a self-etching primer enough?
Etch primers may suit light-duty work, but industrial and CUI systems usually rely on abrasive profiling and high-performance primers.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
