Does stainless steel rust or corrode?
To answer the question about when stainless steel can be oxidized, it is first necessary to explain why common steel oxidizes. Unalloyed steels or low alloy steels, are mainly made of iron.
In contact with a humid atmosphere containing oxygen, iron from unprotected steel can form iron oxide. Oxygen also reacts with the surface of stainless steel. However, since stainless steels must by definition contain at least 10.5% chromium, oxygen reacts preferentially with chromium.
Therefore, instead of forming an iron oxide layer, a dense layer of firmly adhered chromium oxide is formed. This chromium oxide layer is known in the technical field as a passivation layer. It has a thickness of only a few atoms and is not visible to the naked eye. If the layer suffers minor mechanical damage, these will be repaired automatically and immediately in the presence of oxygen.
Why doesn’t stainless steel rust?
Stainless steel is a high corrosion resistance steel, since chromium, or other alloying metals it contains has a high affinity for oxygen and reacts with it forming a passivating layer, thus preventing the corrosion of iron (purely metals stainless, which do not react with oxygen are gold and platinum, and of less purity are called corrosion resistant, such as those containing phosphorus). However, this layer can be affected by some acids, leading to iron being attacked and oxidized by intergranular mechanisms or generalized bites. Some types of stainless steel also contain other alloying elements; The main ones are nickel and molybdenum.
Is stainless steel supposed to rust?
Stainless steels cannot be considered corrosion resistant in all working conditions. Depending on the type of stainless steel, there will be certain conditions in which the passive layer is lost and cannot be recomposed. In that case the surface becomes “active”, and corrosion occurs.
Stainless steel must be cleaned to maintain its good appearance and preserve its corrosion resistance. Stainless steel components will not undergo corrosion under conditions of domestic use, provided that the appropriate type has been selected and normal procedures have been followed.
When can stainless steel oxidize?
Example 1: The passivation layer is responsible for protection against corrosion and the visual appearance of the products. For this layer to be maintained, it is essential to periodically clean the stainless steel. The protective layer can only be formed and maintained on a clean surface.
Example 2: Environmental impurities, such as sulfur dioxides or soot particles, or contact with other materials (defrosting salt, cooking salt or liquids containing chlorine, such as drinking water) can cause lasting damage to the protective layer. If damage of this type occurs, the material may suffer corrosion and rust. The occurrence and progression of oxidation will depend largely on the material selected. As a general rule, it can be said that the higher the alloy percentage of the stainless material, the greater its resistance against corrosion.
Brown oxide deposits that can be found on the surface of stainless steel are rarely corrosion products of stainless steel itself, but generally come from non-alloy steel particles that have reached the surface, where they form rust (foreign oxide).
Some practical examples by way of clarification:
Suppose that on the outside of your house you have a stainless-steel handrail, and that your house is located near a crossroads. When the vehicles brake, brake dust is generated with corrosive material that travels through the air until it is deposited in the steel handrail. In the presence of ambient humidity, these microscopic sized particles can oxidize on the stainless-steel surface. This type of corrosion is called light corrosion. If these “stains” are cleaned in time, the corrosion resistance will be preserved.
Suppose you have a stainless-steel sink in your kitchen. If you leave an office clip in the wet sink at night, the clip will appear rusty the next morning. When removing the clip, you can see a slight stain in the sink in the shape of the clip. These rust residues come from the clip, and can be cleaned without problems with a normal dishcloth.
This corrosion, which is caused by the contact between two different metals, is usually called “contact corrosion” or “galvanic corrosion”. This type of corrosion always attacks the “less noble” metal of the metal pair with more intensity. In the example, that metal would be that of the clip. When stainless steel comes into contact with other usual metals, such as iron or aluminum, it will occupy the position of the most noble metal, so there will be no risk of corrosion as long as it has the passivation layer.
It is essential to use the right tool
Due to the above, when processing stainless steel, it is important to make sure that you use the right tool. Do not use tools that have been used to machine or clean normal steel, as it could contaminate stainless steel. my advice: use a different tool to work stainless steel and common steel.
How do you restore rusted stainless steel?
The type of cleaning of stainless products will depend largely on the degree of fouling. If there is only slight surface discoloration, it will usually be enough to use a conventional sponge, water and dishwashing detergent. After the cleaning process, the surface should be rinsed with clean water and dried.
If the stains are not removed, a sponge combined with a mild abrasive liquid can be used. In this case you will also have to rinse with water and dry. In general, in the private sphere, you should avoid using cleaning products that contain chlorine.
If pronounced damage is detected, abrasive products should be used for material removal. In industrial applications it is usual to strip fine steels in acid pickling baths.
Stainless Steel Resistance In Different Environments
Stainless steels maintain an appearance without substantial change after prolonged exposure to the atmosphere.
In marine atmospheres types 301, 302, 303, 304, 321 and 347 can develop a Scattered surface spotted yellowish. This affectation is reduced in the case of type 309 and is practically eliminated in alloys of type 310 and 316 (The latter presents the best resistance).
Ordinary austenitic stainless steels are practically not attacked by the distilled water.
Prolonged exposure in hot domestic water (60º) has shown that the AISI 304 is highly resistant to corrosion.
The behavior of stainless steel in contact with salt is determined by the exposure conditions: where the water speed is low and there are probabilities that marine organisms or solid materials adhere to the alloy, it can be expected a considerable attack located around or under the attached matter. When The exhibition is carried out under conditions where organisms or other materials solids do not remain attached to the surface of the alloy, the attack on Austenitic is negligible.
Under the most adverse exposure conditions, the alloys containing Molybdenum (AISI 316 and 317) are superior to other compositions.
Neutral and alkaline salts
Austenitic stainless steels are virtually free of salt corrosion. Neutral and alkaline.
The behavior of the stainless in acid saline solutions will depend on the acids released by hydrolysis. Alloys containing molybdenum frequently demonstrate an advantage under the most severe acidity conditions and temperature.
All concentrations of hydrochloric acid attack stainless steels, because this acid easily destroys its passivity.
Type 316 stainless provides useful service at room temperature in sulfuric acid concentrations below 20% and above 85%. Between 20% and 85% on Stainless steel is subject to rapid attack. Alloys containing molybdenum with copper and silicon, together with higher levels of chromium and nickel, provide resistance to attack at temperatures of around 50º C.
Austenitic stainless steels have good corrosion resistance due to nitric acid. In all concentrations and practically all temperatures. Types 304 and 347 are commonly used for this acid to the point of boiling, for higher temperatures types 309 and 310 are recommended.
Normal austenitic stainless steels are widely used to handle lactic type solutions. For hot concentrated solutions it is recommended to use applications of alloys that have molybdenum, such as types 316 and 317.
Foods that contain acids, such as acetic, citric, malic, tartaric and Lactic are processed in equipment made of stainless-steel type 304 or 316. Where Salt is added during food processing plus 316 is recommended.