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Special Report

Surface condition before electropo- Table 1: Salient features of Pickling and Passivation
lishing governs the finished appearance. Contrasting features of pickling and passivation
These processes are also applicable to Pickling Passivation
cast SS. However, the resulting sur- Remove both weld oxide and stainless steel Does not remove weld oxides not
faces will not be as smooth as those attack stainless steel.
on wrought materials, unless they are Typically hydrofluoric, sulphuric (or in steel Nitric acid or electropolishing.
mechanically prepared beforehand. mills, hydrochloric) acid Chelating formulations can have
similar effects to nitric acid.
Most commonly, phosphoric
(H PO ) and sulphuric acids (H SO ) Removes surface breaking sulphides and Removes surface breaking sulphides
carbon steel
and, less rapidly, carbon steel smears.
2
4
3
4
are used in conjunction with a high Changes apperance as it lowers gloss by Nitric and chelating treatments
current density to clean and smooth etching should not change appearance. Electro-
(by metal removal) the surface of the polishing will brighten and slightly
steel. The process preferentially attacks smooth it.
peaks and rounds valleys on the surface
and raises the proportion of chromium Usually mixed with nitric acid – and with Will improve the passive film over a
at the surface. The technique can have rinsing Leaves a passive surface simply pickled one.
a substantial effect on the appearance, passivation process. Pickling process tance of Zinc and Zinc Alloys,
increasing lustre and brightness while removes surface-breaking sulphides Marcel Dekker, New York, USA.
only reducing the measured roughness and carbon steel, whereas passivation 5. Satas, D., 1991. Coatings Techno-
by about 30% (Figure-5). process removes surface breaking sul- logy Handbook, Marcel Dekker,
phides and does not attack carbon steel. New York, USA.
Standards Carbon steel however gets slightly 6. Schweitzer, P.A., 1988. Corrosion
The four categories of chemical treat- smeared in passivation. and Corrosion Protection Hand-
ment are detailed in a number of stan- book, 2nd ed., Marcel Dekker,
dards, but the most commonly used are: Pickling process changes appearance New York, USA.
ASTM A380 Standard Practice of the work piece as it lowers gloss by 7. Leidheiser, R., Jr. 1987. “Coatings, in
for Cleaning, Descaling and Passi- etching. Nitric acid and chelating treat- Corrosion Mechanisms”, F. Mansfield,
vation of SS Parts, Equipment and ments in passivation process however Ed., Marcel Dekker, New York, USA.
Systems. does not change appearance. Electro- 8. Tator, K.B., 1988. “Coatings”,
ASTM A967 Standard Practice for polishing of the work piece will brighten Corrosion and Corrosion Protec-
Chemical Passivation Treatments and slightly smoothen the surface. In tion, Handbook”, 2nd ed.,P.A. Sch-
for SS Parts. pickling, after acid treatment the sur- weitzer, Ed., pp. 453-489, Marcel
ASTM B912 Standard Practice for face of the work piece should be rinsed Dekker, New York, USA.
Passivation of SS using Electro- thoroughly for obtaining a passive sur- 9. Pludek, V.R.m Design and corrosion
polishing. face. Passivation however improves the control, (1977), Macmillan Press
passive film on the surface when com- Ltd., London and Basingstoke.
These very useful documents give pared to pickling process. 10. American Galvanizing Association,
detailed recommendations on many Galvanizing Insights-News Letter.
aspects of selection, application and REFERENCES 11. Delstar Metal Finishing, Inc.,
evaluation of these treatments and are 1. Fontana M.G, Green N.D, Corro- Houston, Texas, USA.
highly recommended. sion Engineering (1967) McGraw- 12. Standard specification for chemi-
Hill Book Co., New York, USA. cal passivation treatments for SS
Features of passivation processes 2. Coatings for Material Protec- parts, American Standards for
Pickling process removes both weld tion. Edited by Makhlouf, A.S.H., Testing Materials (ASTM) Inter-
oxides and SS whereas passivation Woodhead Publishing Ltd., Cam- national, Designation: A 957-05,
process does neither remove weld bridge, UK, (2014). Section: 6.1.1.1, (2007), U.S.A.
oxides nor does it attack SS. Typically 3. Suzuki, I., 1989. Corrosion Resis- 13. Chemistry of modified oxide &
hydrofluoric acid, sulphuric acid or tant Coatings Technology, Marcel phosphate surfaces: Fundamentals &
hydrochloric acid are used in pickling Dekker, New York, USA. Applications, Vol. 36, (2023),
process whereas nitric acid is used in 4. Porter, F.C., 1994. Corrosion Resis- Elsevier, USA.

Chemical Weekly August 13, 2024 193

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