What is passivation?

In chemistry, passivation means treating a surface so that it becomes less chemically reactive. With metals, this usually means that a protective oxide layer forms and protects the base material from corrosion.

Why does stainless steel need to be passivated at all?

Many stainless steels are basically "self-passivating": chromium, and depending on the alloy nickel, reacts with oxygen and forms a thin protective oxide layer. In practice, however, this protection can be impaired, for example by:

• free iron (e.g. from tools, machining, media, contact)
• mechanical processing (grinding, blasting, polishing)
• welding (local heat, heat tint/scale)
• chloride-containing environments (e.g. salts, maritime air, bleach)

What does "passivating stainless steel" mean specifically?

In many applications, passivation primarily removes free iron chemically from the surface, typically with an acidic solution. This keeps the corrosion-resistant alloying elements, such as chromium/nickel, available at the surface and allows them to form a stable protective layer. This definition is reflected, among others, in ASTM A-967 and AMS2700.

Why should passivation be the final process step?

The passive oxide layer can be disturbed again by further processing. Therefore, the sequence is often:
Cleaning → (if necessary: pickling/removing heat tint) → Passivation → Rinsing/drying

This prevents the passive layer from being damaged or contaminated again immediately.

Passivation only works reliably when the preparation is right:

Thorough cleaning

Efficient cleaning is a basic requirement, regardless of whether nitric acid or citric acid is used.

Heat tint and weld areas

Heat tint on weld seams should be removed thoroughly. Suitable measures include grinding, polishing and electrolytic processes, depending on component, surface and quality requirements.

Note on "stainless-steel brushes"

The use of so-called stainless-steel brushes is not recommended, as this can introduce additional iron into the surface.

Ultrasound, rinsing, drying - often underestimated

Three points are decisive for reproducible results:

1. uniform wetting (if necessary, e.g. with ultrasound)
2. thorough rinsing (avoid residues)
3. sufficient drying/passivation time (support protective layer formation)

Stainless steel is considered corrosion-resistant, but not automatically. Corrosion often results from a combination of:

Material selection

Not every stainless steel is suitable for every environment. In maritime or otherwise corrosive environments, "standard stainless steel" can quickly show corrosion problems. Material selection, media, temperature and chlorides all play a role.

Surface & processing

For high corrosion protection, surface quality, cleaning and passivation are decisive. A passivated surface is significantly more resistant than a contaminated or processing-stressed surface.

Contamination: free iron as the main problem

Even small iron particles, for example from tools, media or contact corrosion, can serve as a starting point for rust. This is exactly where CitriSurf® comes in by removing free iron from the surface.

Typical practical cases

* Flash rust: often the result of iron contamination on the surface
* Pitting/rust after laser marking: laser processing can locally change surface integrity
* Mixed assemblies: components with e.g. plastics/aluminium/brass (depending on process and component testing)

Material note: carbon and sulphur content

In stainless steels with increased carbon or sulphur content, selective removal of free iron can make other components remain more "visible". Possible effects:

* grey discoloration (often with carbon-containing grades)
* deposits that can look similar to rust (e.g. with sulphur-containing grades)

In many cases, this can be avoided by:

* highly alkaline pre-cleaning (typically pH > 11)
* clean rinsing and drying processes
We are happy to advise on this on a material- and application-specific basis.

Advanced applications: derouging & removing heat tint

In addition to classic cleaning and passivation, products have been developed for:

* derouging of pure- and ultrapure-water systems
* removal of heat tint on weld seams

These solutions were developed in cooperation with our customers, with a focus on practical requirements.

Passivation with citric acid: typical advantages

Passivation with citric acid is described in standards as an alternative and often offers practical advantages, especially in occupational safety and handling:

* lower hazard compared with strongly oxidizing mineral acids

* often process-reliable and easy to integrate (e.g. as an additional cleaning step)

* citric acid forms water-soluble complexes with iron ions, allowing iron to be dissolved from the surface/near-surface zone

* depending on process control, higher temperatures are possible (typically up to approx. 60 °C, depending on the process), which can accelerate workflows

Nitric acid: typical challenges

Mineral acids such as nitric acid are highly corrosive and impose higher requirements for occupational safety and disposal. In addition, incorrect process control can increase the risk of surface attack/pitting. Disposal is often more complex, especially when metals pass into the bath.

> Important: Suitability always depends on material, surface, component geometry, cleanliness requirement and standard/customer specifications.

Passivation of stainless steel is described in certain standards, especially relevant in aerospace. Important references are:

* AMS2700
* ASTM A-967

These standards describe passivation with citric acid as an alternative to passivation with nitric acid. CitriSurf® products meet these requirements depending on material, component and process design.

REACH & occupational safety: why alternatives are becoming more important

Due to current requirements and changes under REACH, the use of certain substances is being assessed increasingly critically. In many cases, CitriSurf® can be an alternative for passivating stainless steel, especially when occupational safety, environmental protection and process reliability are given greater weight.