
The corrosion resistance of stainless steel is
due to a ‘passive’, chromium rich complex
oxide film that forms naturally on the surface
of stainless steel.
Passivation is defined as the removal of free iron from a stainless steel surface

Stainless steel must have
minimum 10.5% chromium and
maximum 1.2% carbon for this
layer to develop.

To maintain a passive layer, a
sufficient oxygen resource must
be available.
Note : Stainless steel delivered from manufacturing mills will be fully passive.

Even where mechanical
damage occurs such as
scratching or machining,
an instantaneous corrosion
resistant layer is formed if
the steel is exposed to an
oxygen environment.

Service conditions must be
taken into consideration

Tight corners,
mechanical joints, and
poorly finished welds
can all result in
localized reduced
corrosion resistant
areas

Examples include
crevices and pitting
Poorly Finished weld

Heat tints are the result of the thickening of the naturally occurring
transparent oxide layer of the surface of the steel
(pale straw hues to dark blue)

As heat tints are formed, chromium is drawn to the surface of the steel due to
the fact that chromium oxidizes more readily then the iron in steel.

This results in a layer on top and just below the surface that lacks a sufficient
chromium layer consequential reducing corrosion resistance.

In order for passivation to occur, heat tints and any other
surface contamination such as an oxide scale, machining
lubricants, oils, grease, dirt, ect. must be removed using a
passivation solution.

Surface contaminations are dissolved

Oxidation proceeds by Nucleation-( molecules become arranged in a
pattern upon which additional particles are deposited as the crystals grow)

Surface substances form a compound based on metals
characteristics and passivation solution creating the
passive layer

Pickling is a process that removes a thin layer of metal from the surface of stainless steel where surface
chromium levels have been reduced. This cleaning process leads into the steels natural passivation
development.

1.
2.
3.
3 methods for pickling
Tank immersion
Spray
Circulation

Smaller areas such as welded areas may be brushed with pastes or gels and or electrochemical
cleaning.






Pickling =
Dangerous
Extensive Subject
Time-consuming
Messy
Harsh to the
environment

Acid Assisted Passivation

Acid assisted passivation– no
metal is removed but a passive
layer is quickly developed
Before acid passivation treatments are done the steel
surfaces must be…
1. Free from any oxide scale
2. Metal surface layer stripped of heat tined layers –
remove by pickling
3. Surface is clean from any organic contamination,
machining lubricants, oils or grease

Oakite replaces acid
pickling to eliminate the
danger of hydrogen
embrittlement and other
harmful variables

Oakite is an all in one
operation where rust,
paint, phosphate coatings,
lubricants, carbon and
shop soils are removed.

Oakite is applied by tank
emersion or spray washing

There may be circumstances where pickling and passivation
processes occur sequentially (not simultaneously), during acid treatments
involving Nitric acid.
1. Nitric acid alone will only passivate stainless steel surfaces
where as
2. Nitric acid mixed with hydrofluoric acids are usually used for pickling
Industry standard = QQ-P-35C
(ASTM Passivation standard)
has been replaced by consensus
industry standards
1. ASTM A967
2. AMS 2700
Citric Acid

A Citric solution that strips
the free iron from the
steels surface which forms
a water soluble complex
with the iron ions, tying
them up so that they are
no longer able to have a
negative impact.

The Citric Acid bath will
not allow the iron to
precipitate again like nitric
acid is known to do.

Citric acid does not pose
the health and
environmental hazards
that are present with nitric
acid

Citric acid products are non
toxic, non-corrosive and
biodegradable

Produces superior results
at lower overall cost

Cannot be preformed at room temperature

Some citric treatments may be detrimental

Needs to be well understood

Evidence for citric acid as a technically sound
passivation method is scarce.