Stainless Steels and Alloys: Why They Resist Corrosion
and How They Fail*
Dr. Jianhai Qiu
School of Materials Engineering
Nanyang Technological University
Nanyang Avenue, Singapore 639797
ABSTRACT
This paper discusses the corrosion resistance
properties of stainless steels with a focus on why stainless steels resist
corrosion and how they fail under service conditions. It is hoped that the
discussions would help designers, specifiers, engineers and maintenance
personnels understand the corrosion resistance characteristics of stainless
steels and hence aid their specification, design, selection, evaluation and
efficient applications.
Introduction
Stainless steels are a family of special
grade of iron-based alloys that contain at least 11% of chromium in their
composition. As their names suggest, stainless steels can retain the "stainless"
appearance as opposed to the rusty look of common carbon or mild steels. Car
manufacturers often coat steel car body with 6 or more layers of metallic and
non-metallic (paint) coatings to protect the substrate steel. Many of us have
the experience that deep scratched car body, if not touched up promptly, would
become rusty at scratched areas. If the car body is made of stainless steel [1],
protective coatings become unnecessary and scratches on the stainless steel car
body would not lead to rust stain formation. This is because stainless steels
have remarkable resistance to atmospheric corrosion.
The photo showed a 1936 Deluxe Ford Sedan with a
bare (not coated) stainless steel body. Other non-stainless steel components
such as brake, clutch, gears and the engines had to be replaced three or more
times, the surface condition of stainless steel body (after about 60 year's
atmospheric exposure) is essentially the same as when the car left the assembly
line.
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