case study
Material
Approximate Relative Corrosion Behaviour
Carbon Steel (uncoated) Corrodes extremely fast; no passive film; quickly rusts in the presence of moisture, acids, chlorides. Ferritic Stainless 430 Moderately better than carbon steel, but still prone to chloride attack and pitting. Austenitic 304
Much reduced corrosion rate; good general corrosion resistance but susceptible to chloride pitting. Listed as suitable for mild–moderate corrosive environments. Better than 304, especially in chlorides; added molybdenum significantly improves pitting and crevice corrosion resistance. Excellent resistance even in strong chlorides, acids, seawater; far slower corrosion than 304/316. Supported by stainless corrosion tables showing superior performance in highly corrosive environments.
Austenitic 316 / 316L
“High Corrosion Resistance” Super Austenitics / Duplex / Super Duplex
Typical Relative Corrosion Rate Ranges (General Guidance) If carbon steel is compared to different stainless steel grades in different environments the following qualitative ranges are commonly accepted in corrosion engineering and reflected in stainless steel selection tables.
containing grade 316 shows significantly better performance due to the molybdenum and is recommended for seawater, chlorides, and industrial chemicals. Highly alloyed stainless steels such as the super austenitics, and duplex/super duplex grades function excel in severe chemicals and high chloride solutions.
Environment Carbon Steel
304 Stainless
316 Stainless
Atmospheric (rural) Atmospheric (marine)
Moderate rusting
Very low corrosion
Very low corrosion
Low corrosion (better pitting resistance)
Moderate corrosion (pitting possible)
Severe corrosion (rapid rust)
Low
Very low
Fresh water
High corrosion unless coated
Chloride- bearing water
Very high Moderate to high (pitting)
Low to moderate (better pitting resistance)
Acids (mild organic)
High
Low
Very low
Fails rapidly
Moderate to severe attack
Moderate; sometimes acceptable
Acids (chlorides, strong mineral acids)
depending on concentration/ temperature
Corrosion tables (Alleima, Outokumpu) show that carbon steel corrodes quickly in nearly all aqueous acidic and chloride environments. Stainless steel grade 304 is acceptable in many mildly corrosive environments but becomes susceptible in chlorides. The molybdenum
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