Passivate new or worked stainless by degreasing it, soaking it in 20-40% nitric acid at room temperature for 30-60 minutes, rinsing thoroughly and air drying, which dissolves free iron picked up from tools and scale and lets the chromium oxide film re-form clean. Dilute village-made 55-65% acid with water, adding the acid to the water, and do the work outdoors because the fumes are toxic.
Part of Stainless Steel Production
The acid treatment that cleans free iron off finished stainless parts so the protective chromium oxide film can form without contamination.
Passivation Is Mostly Cleaning
The carrier explains the purpose: the acid dissolves free iron from the surface and accelerates chromium oxide formation. Stainless forms its film in air on its own (How Chromium Creates Passivation); what stops it is iron embedded by hammers, files, grinding dust and scale. Each particle of iron rusts and the stain blooms around it, so the surface looks as if the stainless were failing. The acid removes the iron and lets the film form uniformly.
What You Need
- Nitric acid from Nitric Acid, which gives 55-65% acid.
- Clean water for dilution and rinsing.
- An acid-proof tank: glass, glazed ceramic or a suitable plastic; not iron or copper.
- Degreaser, a soap-and-lye wash.
- Tongs or a cradle of acid-proof material, and gloves, eye protection and an apron.
Method: A Passivation Bath
Step 1: Degrease and descale. The carrier asks for a thorough clean: remove all grease, scale and iron contamination. Oil on the surface blocks the acid and can react with it dangerously.
Step 2: Make the bath. Add the acid to the water. See the worked example.
Step 3: Immerse the clean parts for 30-60 minutes at room temperature, per the carrier. Keep them fully submerged and apart.
Step 4: Rinse thoroughly with clean water, several times, then air dry.
Step 5: Test a spot, as below.
Worked Example
The nitric acid article gives 55-65% product. To make 1 kg of 30% bath from 60% acid, by mass: the acid needed is 1 kg × 30 / 60 = 0.5 kg, and the water is 1 - 0.5 = 0.5 kg. For the carrier’s range, 20% needs 1 × 20 / 60 = 0.33 kg of acid and 0.67 kg of water, and 40% needs 1 × 40 / 60 = 0.67 kg of acid and 0.33 kg of water. The carrier does not say whether its percentages are by mass or volume; I have worked by mass and flag the gap.
Checking The Result
The carrier gives a field test: apply a drop of copper sulfate solution (blue vitriol). On good stainless nothing happens; on carbon steel or poorly passivated stainless, copper plates out as a pink-brown deposit. I recall that the standard version of this test acidifies the copper sulfate slightly, and that what it detects is free iron on the surface, not the chromium content; I cannot source either point. A clean result means iron has been removed. It does not prove the steel itself has enough chromium.
Common Mistakes
- Passivating dirty parts. Oil and scale shield the iron from the acid.
- Putting in a part with carbon-steel contamination still trapped in a crevice or seam. The acid may not reach it.
- Mixing water into acid. Always add acid to water, as the neutralization article says for any concentrated reagent.
- Using hydrochloric acid instead. The carrier specifies nitric, and I recall that hydrochloric acid attacks the film, though I cannot source it.
Safety
Nitric acid and its fumes
Nitric acid burns skin and eyes; flush with a great deal of water and do not apply a neutraliser to skin, as Nitric Acid explains. Nitrogen dioxide fumes from a bath are toxic at low concentrations and can cause delayed lung damage, so work outdoors, keep the bath covered between uses, and never use a sealed container. Keep acid away from oil, cloth and wood: acid-soaked organics can ignite. Spent acid is neutralised carefully as in Neutralization before disposal.
What’s Next
- Learn next: Surface Finishing - the finish that decides how clean the surface stays
- Combine with: Solution Annealing - heat treatment that precedes passivation