Stainless steel is specified across aerospace, energy, medical and process industries because it can resist corrosion far better than carbon steel. That corrosion resistance is not “magic” in the alloy, it largely comes from a very thin passive oxide film that forms naturally on the surface.
Passivation is the controlled chemical treatment used to remove surface contamination (especially free iron) and promote the formation of a stable, uniform passive film on stainless steel parts.
In practice, passivation is often the difference between stainless steel that performs reliably for years and stainless steel that pits, stains, or fails prematurely in service.
Passivation treatments are intended to:
These principles sit behind major industrial standards such as ASTM A967 (passivation treatments) and ASTM A380 (cleaning, descaling and passivation practice). However, passivation is not a coating and it does not “add” chromium to the steel. It’s a surface conditioning process which cleans and chemically supports the surface state needed for the passive layer to form properly.
Most passivation procedures look like this:
ASTM A967 explicitly covers nitric acid and citric acid passivation routes and outlines common verification tests such as water immersion, high humidity, salt spray, copper sulphate and other checks for surface free iron.
Both nitric and citric approaches are widely used, and both are recognised in ASTM A967. In general:
In aerospace supply chains, passivation is commonly controlled using specifications such as SAE AMS2700, which exists explicitly to assure removal of free iron / less noble contaminants from corrosion-resistant steels.
Passivation becomes invaluable in industries where corrosion is a direct threat to safety, reliability and operational continuity. In the following sectors, surface condition control is fundamental to ensuring stainless steel components perform as intended over long service lifecycles.
Stainless components used in mechanisms, hydraulic environments and high-humidity service need reliable corrosion performance over long lifecycles. Aerospace specifications such as AMS2700 codify passivation requirements to ensure free iron removal and corrosion resistance consistency.
Why it matters: Corrosion often initiates at localised contamination sites and passivation reduces that risk by removing iron-rich residues and stabilising the surface condition.
In oil and gas, materials may be exposed to H₂S (hydrogen sulphide) environments where risks like sulphide stress cracking must be controlled through materials selection and processing in accordance with widely used standards such as NACE MR0175 / ISO 15156.
Why it matters: In corrosive service, the surface condition and corrosion behaviour of stainless components becomes a reliability and safety issue, not just a cosmetic one.
Valves, pumps, pipework, and process equipment face chemical exposure, cleaning regimes, and shutdown/start-up cycles. ASTM A380 is specifically framed around cleaning, descaling and passivating stainless parts, assemblies and systems. This reflects how common this requirement is in real facilities.
Why it matters: Passivation supports corrosion resistance and helps prevent early pitting, staining or “tea staining” behaviour after fabrication and installation.
In medical applications, corrosion control is essential for both performance and patient safety. ASTM F86 addresses surface preparation of metallic surgical implants with the purpose of improving corrosion resistance.
Why it matters: Consistent surface condition supports corrosion performance in demanding environments and reduces the risk of adverse surface degradation.
If stainless parts will later receive advanced coatings (or plating/strikes), passivation (and pre-cleaning) is frequently part of ensuring the surface is in a controlled state before subsequent processes. Standards such as ASTM A380/A967 exist partly because surface condition drives downstream reliability.
In high-performance industries, stainless steel reliability depends on surface integrity. After machining, welding or handling, free iron contamination can compromise corrosion resistance. This makes controlled passivation essential to restore a stable, protective oxide layer.
At Hardide, acid passivation is delivered as a tightly controlled, repeatable process engineered for aerospace, energy and industrial applications. Available as a standalone service or fully integrated with our advanced CVD coatings, it ensures surfaces are clean, corrosion-resistant and optimised for long-term performance in demanding environments. Click below to find out more about our additional services.