What is corrosion resistance? | Corrosion Resistance of Stainless Steel
What is corrosion resistance?
Corrosion resistance is the ability of a material to resist corroision coused by oxidation or chemical reaction.
Corrosion resistance of stainless steel
Stainless steel is know as a metal which is less prone to rust and this is bacause of the protective action of the film (passive film) existing on its surface.The main component of this film is an oxide (hydroxide) containing chromium, which is closely bonded to the metal to protect the metal. Therefore, in an environment where this film is not damaged, Corrosion Resistance is maintained for a long time.
For example, even if the film is broken in the air, it will be automatically regenerated by
oxygen in the air. This is why it is strong against oxidizing acids such as nitric acid and concentrated sulfuric acid.
On the other hand, in a reducing environment, destruction and regeneration of the film are blocked, so corrosion occurs relatively easily.To improve this, Elements such as nickel, molybden, copper are added to some steel grades to make its passive film strong.
In addition, when foreign matter may adhere to the surface, contact with dissimilar metals, or
there is a temperature or concentration gradient, local corrosion may occur. The surface of stainless steel should be as smooth and clean as possible, bacause corrosion tends to occur especially when the surface is rough or dirty.
Passivity
Generally, when the oxidizing property of the environment becomes strong, metal corrosion is likely to occur, but for
some metals, when oxidizing properties get stronger than a certain level, corrosion may stop.
Such situation is called “Passivation” of the metal, which is caused by the combination of the metal and the environment.In addition, some metals are easily passivated, such as Fe, Ni, Cr, Mo and their alloys (eg: stainless steel).
The following are possible methods for promoting passivation.
(1) Method of immersing in a solution containing nitric acid or other strong oxidants
(2) Method of anodizing a material in a solution includiong an oxidizing agent.
(3) Method of heating materials with low temperature in clean air or oxygen.
Of these, the mainly used method is(1) and there are following methods.
(a) 50% boiling HNO3
(b) 20-40% 60°C HNO3
(c) dipping into 4% HF + 4% CrO 360°C
All for 30-60 minutes.
Also, it is said that the effect is enhanced if the surface is activated before the passivation treatment.
In any case, the thickness of the passivation film is very thin, from several tens of Å to 200 Å, so there may be
problems with the permanent effect depending on the environment.
Intergranular Attack
When austenitic stainless steel is heated to around 400-800°C, carbide precipitates at the grain boundaries and corrosionresistance deteriorates significantly. This is called Sensitization, this is caused by keeping in this temperature range or by
slowly cooling, especially in the heat affected zone during welding.
The following methods are used to prevent this.
(1) Uses low carbon steel with a particularly low carbon content（SUS304L、316L、316J1L）
(2) Add carbide forming elements such as titanium and niobium（SUS321、347）
Reheat the precipitate of carbide to 1000-1150℃ and Cool down rapidly(solution heat treatment)
we recommend the most appropriate type of stainless steel depending on the cause of corrosion.
Also, We might recommend other steel other than stainless steel depending on usage environment and other your request.
If you have any trouble with rust or corrosion, feel free to contact us by an inquiry form.