Atmospheric corrosion is not a unique form of corrosion, but a collective term to denote the corrosion of surfaces in the atmosphere. It may be indoor or outdoor and all corrosion forms can, in principle, be involved. Corrosion of stainless steels is most often induced by halides and mainly chlorides due to their abundance in our environment. When stainless steel is exposed to an aggressive atmosphere it is primarily stained (sometimes referred to as tea staining), but it can also be attacked by localized corrosion with time, particularly at high chloride levels. It should be noted that discoloration of stainless steel is not automatically the result of atmospheric corrosion or even corrosion attack at all; it can be a discoloration from dirt or extraneous rust, not affecting the stainless steel.

Two large application areas can be distinguished where implementation of atmospheric corrosion knowledge is needed: in architectural and structural use. The surface appearance is much more important for architectural use, while superficial corrosion attack can be neglected in structural applications.

Since the corrosion process in the atmosphere is based on wet corrosion it needs a conductive electrolyte for charge transport and dissolution of metal atoms. Therefore, temperature, relative humidity and weather conditions affect the corrosion process on the stainless steel surface, as do polluting gases (i.e. SO2) and other contaminants and aggressive substances that can dissolve into the electrolyte. Moreover, the exposed condition of the stainless steel’s surface is of great importance: sheltering prevents rinsing by rainfall, thus increasing the corrosivity of the environment, and the inclination angle affects the run-off rate. Additional crucial factors are the surface condition and its smoothness. A coarse surface will retain dirt, particles and corrosive chemicals while a smoother surface (BA and 2B) facilitates rinse-off. Finally, one has to take into account that regular cleaning acts decisively for maintaining the metal in ideal condition.


10. Relative pitting corrosion resistance of welded 4401 (316) after different post weld cleaning treatments.

Below there are three tables that present the suitability of various popular stainless steel grades to different environments.

Typical indoor environments	Typical outdoor environments	Suitable stainless steel grades
Heated	Deserts and artic areas (rural)	4016, 4521, 4301, 4372, 2101
Non – heated	Dry environment with low pollution	4016, 4521, 4301, 4372, 2101
Humid environment with low pollution	Coastal environment with low salt concentrations, urban and industrial areas with moderate pollution	4521, 4301, 4401, 2101, 2304
	Polluted urban and industrial environments, coastal areas with moderate salt deposits	4401, 4438, 4439, 904L, 254 SMO, 4565, 2304, 2205, 2507
Swimming pool environments, humid industrial areas with (volatile) aggressive chemicals	Heavily polluted industrial environments, marine atmosphere with high salt concentrations in the atmosphere and a combination of marine and industrial environments	904L, 254 SMO, 4565, 2205, 2507

11. Material selection sheet for some ferritic, austenitic and duplex grades in four different environments: rural, urban, industrial and marine with three different corrosion categories, low (L), medium (M) and high (H), respectively.

Type of outdoor environment and corrosion category

Steel grade	Rural L M H	Urban L M H	Industrial L M H	Marine L M H
1.4003 1.4016 / 430 1.4521 / 444	o x x + o o - - -	x x x x x x - + o	x x x x x x + o x	x x x x x x + o o
1.4301 / 304 1.4401 / 316 1.4439 / 317 LMN 1.4539 / 904L	+ + + - - - - - - - - -	+ + o - + + - - - - - -	o o x + + o - - + - - +	+ o x + + o - - + - - +
1.4162 / 2101 1.4362 / 2304 1.4462 / 2205	- - + - - - - - -	- + o - + + - - -	+ o x + + o - - +	+ o x + + o - - +

+ = optimum choice, o = may be used with precautions, x = not suitable, - = over-specified