How to choose a stainless steel?

Stainless steel against rust: why?

Passive layer of stainless steel: a natural and free reaction

What’s a stainless steel?

Stainless steel is a material which must have no stain, no rust on the surface.

When stain appear, it’s easy to consider it’s a « bad stainless steel ». It’s true or not? Is there a good or a bad stainless steel? Is the great stainless steel which one is not magnetic?

Standard basis

According to EN 10020 (2000) and EN 10088-1 (2014), stainless steel is an alloy of carbon and iron with more 10,5% chromium and other addition elements like nickel, molybdenum, titanium, suffer … Steel’s rust is a combination between iron and oxygen in water or in the air to make iron’s oxides: it’s redox reaction.

With the same reaction, chromium of stainless steel combined with oxygen of environment to form chromium oxides on the metal surface. thus, when chromium oxides recovered all the surface, this layer of chromium oxides is named passive layer and stop the reaction with oxygen and iron, so it stops rust.

The passive layer

The great advantage of stainless steel is that this protection layer makes itself naturally with oxygen of water or of the air. For that, metal surface could be clean. Indeed, this layer is fine, transparent or see-through and brittle. It is less of 10 nanometre (10-6 mm) and this continuity ensures corrosion resistance of stainless steel. The slightest discontinuity or imperfection of passive layer means an unprotected surface like carbon steel surface and an area where iron oxides could be formed, that the way to say, an area where red stains could be appeared.

Stainless steel is synonym of a fine chromium layer’s existence and this remake throughout life. That’s why we are doing the dishes each day: to remake uniform passive layer, we use water to conserve durability of stainless steels pan. So, to remove passive layer of cutlery or flatware is necessary to eliminate food particles which are a discontinuity of chromium-oxygen reaction and remake the protection against rust. A clean surface on stainless steel is to have a good protection against corrosion’s problem.

To make cleaning easier and the formation of the passive layer, the surface condition is significant, in terms of surface roughness. The smoother the surface, the easier it is to clean and form the passive layer to have a good resistance to corrosion. So, we must understand, a polished mirror surface is easier to clean and « to passivate » than a brushed surface with polished scratches and ridges with snag sites. It would be important first to think to have a slick surface to raise corrosion resistance of stainless steel before changing grade.

Different families of stainless steel

Addition elements and grades

If the general principle of passive layer and of surface condition applies in all fields, it would be necessary to choose stainless steel’s grade in relation with using conditions in each environment: the good stainless steel in the true area. In fact, addition elements in iron-carbon-chromium alloy improve metal capacity to resist to aggressive environments, for example, molybdenum, titanium, nickel or niobium. With theses, particular corrosions don’t raise: pitting corrosion, intergranular corrosion, crevasse corrosion … 

Stainless steel’s families and using

Several alloy’s elements improve mechanical or wrought properties too. They have an important rule for material’s microstructure.

They also form grades distributed in 4 families: ferritic, martensitic, austenitic and duplex. Metallurgical structure obtained with addition of chemical elements give the name of each family and it is the percentage of alloy elements which characterize each grade. The diagram below compares corrosion resistance and mechanical properties for each stainless steel family.

*PH is a family, either austenitic or martensitic, which is characteristic because of Precipitation Hardening.

This summary diagram includes all the corrosions that can affect stainless steels. Corrosion resistance is not the same for all corrosions. There is a large number of corrosions and they depend on the family of stainless steels and the aggressive environment. These include the susceptibility of austenitic stainless steels to stress corrosion or the absence of pitting (pitting corrosion) of Duplexes.

Thus, each family has particular characteristics that give it privileged areas of use. At the origin of a piece of equipment, the combination of "Mechanical Properties" and "Corrosion Resistance" is the subject of a study to guarantee the durability of the metal in use. Each grade has its own characteristics to meet the demands of manufacture and use. For more details, the EASYINOX training programs include these points

Let's look at an example application: 

In the boating industry, the screws inside a boat are made of standard austenitic stainless steel 1.4301 or X5CrNi18-10 or AISI 304 or A2 (designation for bolt screws). Those on the deck of the boat are made of austenitic stainless steel 1.4404 or X5CrNiMo17-12-2 or AISI 316 or A4, which is more resistant to corrosion: they contain molybdenum.

Norme EN 10088-1 (2014) : https://www.boutique.afnor.org/fr-fr/norme/nf-en-100881/aciers-inoxydables-partie-1-liste-des-aciers-inoxydables/fa173619/44456

Attention is drawn to the fact that the designation AISI 304 or AISI 316 may correspond to an American standard with slightly different analytical ranges (minimum and maximum) for elements such as carbon, chromium and nickel in particular. 

For the specific designation of stainless steels screws, the EN ISO 3506-1 (2010) standard imposes different analytical ranges, especially for nickel.

In the case of boats, outdoor application, facing sea spray, requires improved corrosion resistance. Whatever the part (sheets, tubes, screws), the related standard takes into account this stress: corrosion, and imposes additional elements bringing the improvement of the property expected in use: corrosion resistance.

Other examples could demonstrate the same normative adaptations to obtain mechanical properties that make it easier to manufacture parts. Nickel facilitates the stamping of austenitic stainless steels (we speak of DDQ grades), sulphur improves the machinability performance of this family of stainless steels...

Basis on stainless steel equipment

When the choice of alloy is defined according to the environment and the conditions of use, the manufacture of the part will generate injuries such as shocks, impacts or scratches on the surface of the metal. The passive layer does not have the same elongation properties as that of the metal it protects. Its thickness of a few angstroms does not protect the part from mechanical contact. It is altered by friction, cracked by deformation and polluted by polluting impacts: it must be stainless steel

Faced with this type of situation, it is important for stainless steel to regenerate the passive layer. Stainless steel equipment manufacturers can have 2 opposing positions:

• manufacture the part without worrying about the degradation of the passive layer and finally before shipping, regenerate a passive layer without pollution and discontinuities, by an appropriate chemical treatment: this is the pickling - passivation treatment,

• manufacture the part by making every effort to avoid pollution and discontinuities, to preserve it as much as possible throughout the manufacturing process and before use, let the metal consolidate the naturally formed passive layer. In this case, it must be considered that the welding joints are chemically treated locally.

Feedback shows that both positions lead to a conclusive result when the stainless steel grade has been well chosen for the environment it faces in use and for its manufacturing process. Keep in mind that, for any shade chosen, it is often the environment of use that dictates the level of quality of the passive layer. A chemical passivation pickling treatment ensures a superior passivation quality

In particular, it strengthens the chemical bonds between chromium oxides. Thus, by performing a corrosion test, the naturally passivated part will have a lower corrosion resistance than that of the same chemically passivated part (same grade, same manufacture and same surface finish).

The shade has been chosen perfectly, it has been manufactured according to the rules of the art, it is delivered to its user to be used in a sustainable way. It is therefore also the responsibility of the user that will come into play. It supports the maintenance and regeneration of the passive layer throughout the life of the equipment. It takes care of the room's natural protective reaction, as we do for dishes: restoring the passive layer after each meal.

Observe the instructions for use: from the boat with the fittings to the stainless steels plate of the housewife, from the municipal swimming pool to the street furniture, it is a question of regular maintenance of the equipment.

Thus, we should not talk about good or bad stainless steel, we should rather talk about "good or bad choices" of stainless steel or "good or bad users". From the designer to the user, the choice of stainless steels and the maintenance of the passive layer are vectors of sustainability.