Stainless steels cannot
be passivated unless the steel surface is clean and free from contamination and
scale from welding operations.
Passivation improves
the corrosion resistance properties of components made of
precipitation-hardened, austenitic, and martensitic steels. Passivation
treatments improve the surface condition of stainless steel by dissolving iron
that has been imbedded in the surface during forming or machining. If allowed
to remain, the iron can corrode and give the appearance of rust spots on the
stainless steel.
Almost any stainless
steel that you can buy has been rolled, machined and/or manipulated with carbon
or tempered steel implements. This means that, in general, all of the stainless
steel that you will ever buy off the shelf or re-manufacture will have an
appreciable amount of steel worked into its surface. Without Passivation iron
or iron oxide exists on the surface of stainless steel. When any machining or
fabrication operations are performed, the condition is worsened. Aside from
iron being deposited by tooling, it is also being entrapped underneath the
“skin” created by the mechanical operation. In the metal industry this surface
impregnated steel is referred to as "free iron".
Nitric acid alone can
be used to remove light surface iron contamination after which the acid
facilitates the passivation of the cleaned steel surface.
Passivation, which
consists of immersing stainless steel components in a solution of nitric or
citric acid without oxidizing salts, will dissolve the imbedded iron and
restore the original corrosion-resistant surface by forming a thin, transparent
oxide film.
Features
• Maximum stainless
steel corrosion protection without plating.
• Superior clean
surface.
• Eliminates iron
contamination reactions with other materials.
• No rust
discoloration.
• Surface prepared for
priming and painting.
Essentially,
passivation is the removal of free iron from the surface of the steel. This is
performed by immersing the steel in an oxidant, such as nitric acid or citric
acid solution. Since the top layer of iron is removed, passivation diminishes
surface discoloration. While passivation does not affect the thickness or
effectiveness of the passive layer, it is useful in producing a clean surface for
a further treatment, such as plating or painting. On the other hand, if the
oxidant is incompletely removed from the steel, as sometimes happens in pieces
with tight joints or corners, then crevice corrosion may result. Most research
indicates that diminishing surface particle corrosion does not reduce
susceptibility to pitting corrosion.
Passivation refers to
the chemical neutralization of the surface - increasing corrosion resistance.
This free iron corrodes
easily. The corrosion process of iron is a very aggressive reaction. This
reaction will in almost all cases start corrosion in the stainless steel. Once
started, the corrosion of the stainless will continue to take place without the
presence of free iron.
The term
"Passivation" is used widely in the stainless steel processing
industry. Many people refer to this
oxide surface as a chrome-nickel oxide.
In lay terms, the
passivation process removes "free iron" contamination left behind on
the surface of the stainless steel from machining and fabricating. These
contaminants are potential corrosion sites that result in premature corrosion
and ultimately result in deterioration of the component if not removed. In
addition, the passivation process facilitates the formation of a thin,
transparent oxide film that protects the stainless steel from selective
oxidation (corrosion). So what is passivation? Is it cleaning? Is it a
protective coating? It is a combination of both. This process is known as passivation. Visit their website to find out more
ASTROPAK.COM
Equipment and
precautions. Passivation should only be performed by trained, experienced
technicians familiar with the potential hazards associated with the science. Safety
practices must be fully understood when handling passivation chemicals. Special
boots, gloves, aprons and other safety equipment must be used. Tanks, heaters
and ventilation, as well as baskets and racks must be appropriately engineered
to perform the process. Iron or steel parts or equipment must never be
introduced to the process, or the results can be devastating. Furthermore, in
order to comply with EPA requirements, the necessary water and air permits and
treatment capabilities must be in place. The days of mom-and-pop shops
performing passivation in a stone crock in the back of the shop are gone.
Specifications and
verification testing. There are a few generally accepted industry
specifications available for reference when choosing a passivation process.
They offer time, temperature and concentration information and subsequent
testing requirements to validate the effectiveness of the process. Many large
corporations have developed internal specifications to control their unique
requirements regarding passivation and verification testing. Regardless of the
situation, it is usually prudent to reference a proven procedure when
requesting passivation. By referencing a specification, you do not have to
reinvent the wheel. By taking advantage of the experiences of others, both
successes and failures, you can eliminate much of the guesswork that would
otherwise accompany a new process.
So how do you get the
performance you have paid for from high-dollar stainless steel alloys? It boils
down to a basic understanding that the passivation process is both an art and a
science, and that machining, fabricating and heat treating practices can
substantially affect the corrosion resistance of the component. Passivation
will enhance the corrosion resistance of stainless steels, but to realize the
maximum performance from these high-tech alloys, all parties involved with
manufacturing must understand their responsibility in maintaining the integrity
of the material throughout the process.
Stainless Steel Maintenance Tools
The Introduction of Stainless Steel Products
A Quick Way To Know How To Choose The Correct
Stainless Steel
To find
out more about Passivation Click here : http://www.astropak.com/ultra-pass-passivation.php
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