Pickling and passivation: choose the method & prove it

At a glance

This blog shows which pickling method suits which component, what typically causes corrosion on stainless steel, when pickling, passivation or both is needed, and how the result can be verified. The basics, why stainless steel can rust at all and how the passive layer works, are explained in the linked article.

 

Which method for which component?

Pickling is not just pickling. Depending on the shape, size and accessibility of a component, a different method is used.

Method	Typical components	Characteristic
Immersion pickling	sheets, profiles, complete constructions up to XL format	uniform, intensive action in the bath
Spray pickling	large tanks, black-and-white constructions, parts with gap problems	flexible, also for partial areas and bulky geometries
Circulating pickling	pipework and closed systems	pickling agent is pumped through the inside

Which method is right depends strongly on the construction. For a sheet the immersion bath is ideal, for a welded large tank the spray method, for a pipe system the circulation method. Methods can often be combined as well.

 

Typical triggers of corrosion on stainless steel

Most corrosion damage on stainless steel is caused not by the material itself, but by contamination from fabrication and assembly. The most common causes are:

• heat tint and scale after welding
• extraneous rust from tools, brushes or abrasives made of unalloyed steel
• embedded grinding dust and foreign particles
• site contamination, e.g. spark fly from cut-off grinders
• deposits in gaps where the oxygen for the passive layer is missing

Each of these points creates a local weak spot where pitting or crevice corrosion can begin. Pickling removes the contamination, passivation then restores the full-surface protection.

 

Pickling, passivation, or both?

As a guide: if visible heat tint or scale is present after welding, the combination of pickling and passivation is the right way. If the surface is already mechanically processed and free of scale, passivation alone is often enough. If extraneous rust or incipient corrosion is already present, pickling must be done first, before passivation. Where tools made of unalloyed steel have been involved, treatment is generally advisable, because introduced iron particles would otherwise continue to rust.

 

Industries and environments with high risk

Corrosion protection is particularly critical where stainless steel is permanently stressed. This includes beverage and food tanks with their cleaning cycles, pharmaceutical and clean-room applications, swimming pools and chloride-containing atmospheres with an increased risk of stress corrosion cracking, fastening and component technology with large quantities as bulk material, as well as tank, plant and environmental engineering. Especially in chloride-containing environments, an intact passive layer determines service life.

 

Proving quality: how passivity is verified

Proper treatment can be documented. Several stages are common:

• visual inspection for heat tint, pickling residues and extraneous rust traces
• detection of free iron particles, e.g. via a copper sulphate or ferroxyl test
• for higher requirements, a corrosion or salt spray test
• orientation towards established passivation standards

At INOX COLOR every step is documented and organised in line with ISO 9001. This makes it traceable that the surface is metallically clean and passivated, not just visually clean.

 

How your order is handled at INOX COLOR

Visual inspection and degreasing are followed by pickling in the appropriate method; the component is then rinsed with high pressure until free of acid. After that it is passivated and rinsed again, and finally dried. On request we combine the treatment directly with electropolishing or mechanical processing. Components up to 6 x 2.5 x 2.5 m and small parts as bulk material are not a contradiction here, but everyday business.

 

Further details

More detailed information on pickling and passivation can be found here: Pickling and Passivation

 

 

Frequently asked questions

How is successful passivation proven?

Through visual inspection and tests for free iron particles such as the copper sulphate or ferroxyl test, supplemented by a corrosion test if required. The results are documented.

 

What is extraneous rust and how is it removed?

Extraneous rust arises from iron particles from tools or abrasives made of unalloyed steel that continue to rust on the stainless steel surface. Pickling and passivation remove these particles and restore the protection.

 

Does pickling attack the base material?

Pickling removes a very thin, deliberately chosen layer to dissolve contamination. Passivation itself does not attack the base material, it only strengthens the protective layer.

 

How long does the passive layer last?

With an intact surface and sufficient oxygen, the passive layer renews itself. Mechanical damage, deposits or high chloride exposure can, however, destroy it locally; renewed treatment is then advisable.

 

Can already installed or very large structures be treated?

Yes, large tanks and pipe systems can be treated via the spray and circulation methods, in many cases also partially.