INOX – Steel processing guide
5- RESISTANCE TO CORROSION
It is always very aleatory be able to provide, in general, the behavior in time of a given metallic material if put in contact with a certain environment.
Stainless steels, due to their chemical composition, have the possibility of self-passivation, as seen at point 1) and to be able to meet the most disparate conditions of aggression.
There are many parameters which favor the triggering of a corrosive phenomenon:
- aggressive agent concentration;
- aggressive agent temperature;
- speed of the fluid on the walls;
- surface finish of the metal;
- etc.
Normally, however, the two values more determinants to note are the concentration and the temperature of the corrosive substance; that’s why when choosing a steel according to the environment in which it will have to work, you need to know, if possible, at least these two parameters.
In general, the best behavior towards the corrosive phenomena, is offered by the austenitic steels, in particular those related to the chromium-nickel-molybdenum, which have a particularly resistant passive film. The order then come the ferritic and martensitic steels which are those with the lowest levels of chromium.
To avoid inneschino corrosion on steel, in addition to suitably choose the type of service as a function of the material to which it is intended, it is also good to follow some precautions during processing and the mass in work.
It is necessary in the meantime, both during storage of the products, both in the phase of transformation during machining operations, both during installations, avoid contamination with less noble materials, such as those caused by steel common.
The possibly contaminated steel, even in the presence of an aggressive very bland, may become stained and start to localized corrosion phenomena.
The unions with the filler material have to be performed with the electrodes shown for the base material; while the junctions carried out by mechanical elements must provide that the materials constituting the connecting members are of steel or of equal nobility (eg. monel, etc.). This is to avoid that inneschino galvanic corrosion due to the different nobility of the metals brought into contact.
We must not use, on finished components, cleaners or strippers who have high percentages of chlorinated substances (for example, you should not use hydrochloric or commercial muriatic acid).
Sometimes in order to effectively clean the surfaces dell’inox simply normal soap and water or water and soda.
In cases where the dirt is much more resistant or where it is necessary to proceed to pickling (for example, the weld beads) or decontamination (in the case of ferrous contamination), it is possible to use appropriate products in paste to be used in a localized manner onto the area to treat.
Here you are shown, schematically, the most common corrosion on steel and recommended installation methods to avoid collision with a proper choice of the type of steel to be used and with practical tips to follow.
- 5/aPITTING
How the material looks | Why it happens? | How to avoid it |
---|---|---|
On the surface can be seen dotting laying flocks. | The passive layer tears apart locally due to strongly activating substances (for example chlorides). | Use steel with more resistant passive layer. In the order: Cr steel; Cr-Ni; Cr-Ni-Mo; etc. Avoid locations with large quantities of chlorine ions and halides in general. |
- 5/bCREVICE
How the material looks | Why it happens? | How to avoid it |
---|---|---|
In the interstitium matching can be seen corrosion stains and pitting. | The passive layer tears apart locally due to poor oxygenation in the interstitium, with the presence of a corrosive substance. | At an early stage, eliminate the gap sealing it. Use steel with more resistant passive layer. In the order: Cr steel; Cr-Ni; Cr-Ni-Mo; etc. |
- 5/cINTERGRANULAR
How the material looks | Why it happens? | How to avoid it |
---|---|---|
Crumbling grains in intergranular sense. | The thermal alteration (450÷850 °C for austenitic steels) for a certain period of time (welding, heat treatments, in operation service), causes a precipitation of chromium carbides and, in the presence of a corrosive substance, it can start the corrosive attack. | In the case in which it is not possible to eliminate the thermal alteration, use stainless steels with a low carbon content or stabilized steels (titanium or niobium). |
- 5/dSTRESS CORROSION
How the material looks | Why it happens? | How to avoid it |
---|---|---|
Orthogonal cracks develop respect to the direction of tensioning of the material. Cracks can carry in both granular directions. | The simultaneous action of a mechanical stress and of a chemical attack may create the trigger of the crack, especially of austenitic structures. | Where it’s not possible to avoid the mechanical action or the chemical action, use steels with a partial ferritic structure (austenitic-ferritic) or a completely ferritic structure. |
- 5/eGALVANIC
How the material looks | Why it happens? | How to avoid it |
---|---|---|
The less noble material (for example, zinc, aluminum, carbon steel), if in contact with the stainless steel, can undergo a corrosive process (rust, craters, etc.). | Stainless steel, moved to the cathode end of the galvanic scale of the elements, if put in contact with an anodic element (less noble), creates the conditions to develop, in a certain environment, corrosion on the less noble metal. | Couple the stainless steel with a material of equal nobility (especially in case of smaller size compared to the stainless steel component), or interrupt the metal continuity between the two different materials with inert elements (rubber, Teflon, etc.). |
- 5/fHOT OXIDATION
How the material looks | Why it happens? | How to avoid it |
---|---|---|
Layers of dark oxides and anchored to the surface. | Due to the excessive metal thermal alteration: heat treatment, welding, or in operation. Each steel has its own temperature limit, beyond which occurs in hot oxidation. | If it’s not possible to limit the thermal alteration, use a refractory steel with high chromium content, both of ferritic series or austenitic series. |