Corrosion of stainless steel?
An important factor influencing corrosion resistance is the smoothness and purity of the surface. Even minor surface irregularities can become the seed of corrosion. The first sign of corrosion of the general steel is usually the matting of its surface.
The reasons for changes in the appearance of the surface can be as follows:
- application of the steel species in a more aggressive environment than the projected
- too rough surface to stop deposits and dirt
- design errors causing gaps and pockets in which water and contaminants accumulate
- contamination of the surface of stainless steel with iron particles
- transport and as a result of the use of inappropriate tools or abrasives in the manufacture or assembly of
In coastal areas and in conditions of polluted urban and industrial atmosphere, austenitic chromium-nickel steel with molybdenum is necessary.
This applies in particular to the equipment of bathing areas, both open and indoor, where corrosion is facilitated by ambient humidity and increased water temperature, and in particular the presence of chlorides used for disinfection.
Steel corrosion resistance depends on three factors:
1. Chemical composition
the most important thing in this case is the content of chromium, nickel, carbon, molybdenum, copper, manganese, nitrogen, titanium, niobu and tantalum.
The basic element of these steels is chromium. Introduced into steel in quantities greater tha[%]n 13 causes a spike in electrochemical potential. It follows that corrosion resistance occurs only with a content above 13 ch[%]romium.
Chromium steels are resistant to corrosion in oxidizing environments e.g. nitric acid, but they are not resistant to reducing environments e.g. hydrochloric acid or sulphuric acid. At high temperatures, the minimum chromium content for corrosion resistance increases to 20 [%].
The second most important alloycomponent of corrosion-resistant steels in addition to chromium is nickel, which increases the resistance of steel to many corrosive environments, especially sulphuric acid, solutions of neutral chlorides like sea water. Steels containing nickel are not resistant to gases containing sulfur compounds at elevated temperatures due to the formation of nickel sulphide. Coal, on the other hand, worsens corrosion resistance. Steel is severely reduced by corrosion resistance if carbon is in the form of carbides.
2. Steel structures:
In corrosion-resistant steels there are different structures, so steel can be:
ferritic stainless steel
austenitic stainless steel
martenzith stainless steel
These can always have a single-phase structure e.g. ferritic or two-phase ferrite, e.g. ferritis-austenitic.
The highest corrosion resistance is shown by austenitic constants afterwards ferritic, and the lowest martensitic. Single-phase structures have greater corrosion resistance.
Greater resistance of single-phase structures should be prescribed much more favourable conditions for the formation of a passive state and to maintain its durability and continuity. The probability of local cells forming in single-phase steel is very low. The appearance in single-phase steels of additional components in the structure always leads to a decrease in corrosive resistance.
3. Surface status:
Smooth surface steels are always more corrosion resistant than steel with significant roughness.
In corrosion-resistant steels, the main alloy component is chromium. The chromium additive seeks to create chromium in the carbide structure, which crystallizes in the hexagonal network. Corrosion resistance of steel is associated with the ability of steel to pass. Under the name of passivation we mean increasing the resistance of the metal to corrosion by oxidizing its surface. It is assumed that there is a sealed and highly adhered oxide film on the surface of the passive metal, which protects the metal from the influence of the surrounding environment.