Page 191 - CW E-Magazine (13-8-2024)

Special Report

plots) of the metals to be characterized oxide that covers the SS surface and chromium level on the surface.
(12, 13). protects the underlying iron from rust- Chelating – Chelating agents are
ing. The purpose of passivation of met- chemicals that remove surface con-
In the mid-1800s, chemist Christian al is to augment and optimize formation taminants.
Friedrich Schönbein discovered the of the chromium oxide layer. Electropolishing – this involves
effect of passivation. After dipping iron electrochemical treatments that re-
in concentrated nitric acid, he found Immersion of SS in an acid bath move impurities and have the added
that the iron had little or no chemical dissolves free iron from the surface benefit of smoothing and brighten-
reactivity compared to iron that did while leaving the chromium intact. The ing the surfaces. Electropolishing
not receive the treatment. His name acid chemically removes the free iron, is an electrochemical process. It
for that lack of chemical reactivity was leaving behind a uniform surface with removes gross weld scale and heat
the “passive” condition. As passivation a higher proportion of chromium than tint, but is not intended to brighten
of SS with nitric acid became a wide- the underlying material. Upon exposure or smoothen the surface.
spread practice in the 1900s, environ- to oxygen in the air after the acid bath,
mental and safety issues with nitric acid the SS forms the chromic oxide layer Both pickling and passivation solu-
became more apparent. Research done over the next 24-48 hours. The higher tions can employ dangerous acids that
by the Adolf Coors brewing company proportion of chromium at the surface can damage both the operator and the
in Germany identified citric acid as an allows for the formation of a thicker, environment if not handled correctly.
effective alternative, and in the 1990s, more protective chromium oxide layer. Pickling acids for SS are highly corro-
many manufacturers began to adopt Removal of free iron from the surface sive to carbon steel. It is essential that
citric acid as a safer and more environ- removes opportunities for corrosion to all acids are thoroughly removed by
mentally friendly alternative to nitric start. The resulting passive layer pro- rinsing the component after completing
acid. vides a chemically non-reactive surface the process. Residual hydrofluoric acid
that protects against rust (Figure-4). (HF) will initiate pitting corrosion. If
Today, the industry standards for there are crevices, it may be of benefit
surface passivation offer three types of Passivation processes and chemi- to neutralize the acid with an alkali
passivation: nitric acid, nitric acid with cals for treating metals can be grouped before the final rinsing step.
sodium dichromate, and citric acid. into four categories as explained below:
Choice of which chemical to use of- Pickling – this process uses acids Pickling
ten depends on customer requirements. that remove impurities (including Pickling is the removal of any high
Each passivation type has its own high temperature scales from wel- temperature scale and any adjacent low
advantages and disadvantages. ding or heat treatment) and etch the chromium layer of metal from the sur-
steel surface. face of SS by chemical means.
Why passivate SS? Passivation – this process uses oxi-
Passivation is a post-fabrication dizing acids or chemicals, which Where the steel has been heated by
best practice for newly-machined remove impurities and enhance the welding, heat treatment or grinding to
SS parts and components. Benefits
include:
Chemical film barrier against rust;
Extended life of the product;
Removal of contamination from
product surface; and
Reduced need for maintenance.
Working principle of passivation
SS is an iron-based alloy, typically
composed of iron, nickel and chromium.
SS derives its corrosion-resistant prop-
erties from the chromium content.
Chromium, when exposed to oxygen
(air), forms a thin film of chromium Fig. 4: Microscopic view of passivation layer on Stainless Steel (oxide layer of Cr O )
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Chemical Weekly August 13, 2024 191

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