Our product. Stainless steel for the future.

Stainless steel has repeatedly conquered new fields of application. Today, this material is used in practically all areas of life due to its hygienic and corrosion-resistant properties. Be it in architecture, food equipment or with chemicals there is no situation that cannot be resolved using our materials. The materials most used today due to their ease of welding, corrosion resistance and durability are grades 1.4307, 1.4541, 1.4571, 1.4432 and 1.4404. However, special alloys, such as 1.4462 and 1.4539, are also constantly opening up new fields of use.

In order, however, to fully exploit the advantages of austenitic materials it is necessary to carefully consider their drawbacks as well. Pitting, cracking and stress corrosion, which can occur in combination with acidic media and substances containing chloride, limit their use. However, with careful selection it is possible to find the appropriate material for every kind of purpose. When comparing costs the cheaper quotations should always be checked to see if they are actually offering the same as the more expensive one. For example, is a pipe made of the same material but not as extensively tested or manufactured in a different way? Is it HF-welded or not designed for the same purpose as a pipeline pipe?

Alloy elements

Steel is regarded as corrosion-free if it contains more than 12% chromium. In this case the steel attains an invisible, thin coating of chromium oxide, which passivates the surface and reduces the risk of it being damaged by corrosion. The higher the level of chromium content, the greater the resistance against more corrosive media. In addition to chromium, molybdenum and nitrogen also have an influence on the steel’s resistance. Nickel, on the other hand, primarily influences the structure and the mechanical properties. The risks when using steel in media containing chloride arise above all at high temperatures and with low ph values. In such cases the following types of corrosion can occur.

Corrosion problems

Despite the durability of the steels problems can arise during use. Below we describe the main forms of corrosion.

Pitting
Pitting occurs when the passive coating collapses as a result of local corrosion. It leads initially to the formation of pits resembling pin pricks and through their growth to pitted areas, which in certain cases can be like a drill hole with a diameter of 0.5 to 2mm. In the worst-case scenario these holes can create a leak in a 2mm pipe within 2-3 weeks. The remaining surfaces stay practically completely undamaged, as here the passive coating still exists. The risk increases in the presence of above average salt concentrations, nitrogen content and temperatures, as well as with a falling ph value.

By adding chromium and molybdenum, resistance against pitting is significantly increased. The relative influence of these elements on the level of resistance is expressed by the following pitting index formula.

W = % Cr + 3.3 x % Mo + 16 x % N

The higher the pitting resistance equivalent number (PREN), the greater the resistance against corrosion. These figures are only indicative values and cannot be generally used to evaluate the suitability of a material. There are several methods for evaluating a grade of stainless steel, with the most commonly used being the ASTM G48, which is performed in 6% FeCl_ solution and at various different temperatures. Another method is performed in NaCi solution and is based in part on the ASTM G61. With both methods the results are designated as the Critical Temperature at which pitting begins.

Cracking corrosion
This form of corrosion always occurs in narrow gaps filled with fluids, e.g. under gaskets, flanges and sediment. The medium is generally depleted in oxygen, so that a passive coating can no longer form. It also readily occurs at spots containing impurities, such as scale residues, encrustations or reaction remnants on the material’s surface. In this respect the designer can avoid such spots in advance and the firm processing the product can itself have a big influence by adhering to the prescribed surface treatment, e.g. pickling after welding.

Stress corrosion
Stress corrosion is the consequence of the interaction of corrosion and tensile stress. This type of corrosion is termed the Achilles heel of austenitic steels. The medium triggering the corrosion in many cases is water that contains chloride. At temperatures of up to c. 60 degrees this form occurs relatively rarely. However, the risk increases greatly when the temperature rises above 60 degrees and at 125 degrees and above can even occur in very concentrated alkaline solutions. Tensile stresses within the material also increase the risk.

Improved resistance is achieved by using AST 904L or 254 SMO. Duplex steels, such as 1.4462, also have a much better resistance than conventional alloys against this form of corrosion. We offer all kinds of pipe union parts in these grades of steel made in our own production facility.

Corrosion risks from welding

Welded areas that have not been thermally treated or were welded without any over-alloyed additional material have a lower resistance to corrosion in many media than the base material. The following forms of corrosion can occur on stainless steels in the area of a weld:

  • Pitting due to liquation
  • Pitting due to weld oxide
  • Selective ferritic corrosion
  • Intercrystalline corrosion

 

Pitting due to liquation
In the case of molybdenum-alloyed steels that were neither welded using any over-alloyed additive nor subjected to any form of thermal treatment resistance in the seam goes down if the medium in question contains chloride.

Pitting due to weld oxide
During welding oxide generally forms on and next to the weld seam. The surface under the oxide is more susceptible to corrosion, because it is depleted in alloying elements, especially chrome, which was bonded to the oxide. Here too the greatest risk is in watery solutions containing chloride and then in the form of pitting. This risk can be lowered if after welding the surface is pickled with extreme care. Latest findings show much better levels of resistance compared to blasting, grinding or polishing.

Selective ferritic corrosion
In certain acidic media the corrosion resistance of weld seams on austenitic stainless steels goes down the higher the level of ferrite content, because, among other factors, different alloying elements distribute themselves irregularly between the ferrites and austenites. The ferrite content can be lowered using thermal treatment or through the choice of additional material. As a consequence of thermal treatment the resistance against pitting also increases. A low level of ferrite content is an indication that the weld seam was thermally treated. This information can also be obtained using a simple hardness test. Where values are over 300 HB a more precise test should be done.

Intercrystalline corrosion
This form of corrosion, which can occur in the thermally treated areas of steels with a carbon content of over 0.05%, occurs more rarely in thin-walled structures, as here cooling is quicker after welding and thus no chromium carbide deposits are able to form. In the case of the materials usual in Germany titanium or niobium gets added to the solidifications, while in the case of other alloys a carbon content of less than 0.030 % is aimed for right from the smelting stage. Both methods increase the level of resistance.

Conclusion

When using our materials taking account of the guidance above it is possible, despite sometimes significantly higher purchasing costs, to achieve major savings as a result of long service life and low maintenance. Doing the comparison is always worthwhile and we will gladly help you in choosing the appropriate material. Should we not have the answers, we will undoubtedly know a suitable expert to whom we can introduce you. Make certified materials - which generally come from TÜV-approved factories - a high priority! Sulz Stainless Steel Services seeks out suppliers for you within your own high ISO 9002 specifications. This safeguards the level of quality and gives you the certainty of always using a trouble-free product.

Norms and categories

The most important norms and standards for rust-free stainless steel are:
EN 10088 General use
EN 10095 Heat-resisting stainless steels
EN 10272 Stainless steels for pressure purposes

All rust-free steels get split into categories, i.e. into martensitic stainless steels and precipitation hardenable steels

These steels contain 12 to 19% chromium and 0.08 to 1.2% carbon. They may also contain nickel and molybdenum, as well as additional elements such as copper, titanium or vanadium. This group of materials combines good corrosion resistance with mechanical properties that match those of alloyed non-corroding steels. The properties are achieved through hardening and annealing. Examples of materials in this group are materials 1.4021, 1.4034 and 1.4542.

Ferritic stainless steels
These alloys are iron / chromium / molybdenum, with the percentage of chromium fluctuating between 10.5 and 28% and that of carbon not exceeding 0.08%. These steels generally contain no nickel and are ferromagnetic. Examples of materials in this group are materials 1.4016, 1.4113 and 1.4510.

Austenitic stainless steels
These are the materials that we predominantly keep in stock. They contain at least 17% chromium and at least 7% nickel. Additional elements such as molybdenum, titanium, niobium etc. Their mechanical properties in relation to tensile loading are generally fairly average, but in relation to corrosion resistance usually above average. Examples of materials in this group are materials 1.4307, 1.4541, 1.4571, 1.4404 and 1.4435.

Alloy elements

Listed below are some of our products’ most important alloy elements and their effect.

Chromium
Ferrite former - at a minimum content level of around 12% leads to passivation of the steel and thus represent the main alloying element for stainless steels.

Molybdenum
Increases the corrosion resistance of stainless steels in media that have a reducing effect. In the absence of halogen ions resistance to pitting is also particularly improved. In addition, it enhances thermal resistance.

Silicon
Ferrite former - improves resistance to scaling. Higher levels of silicon content improve corrosion resistance under exposure to certain conditions (e.g. highly concentrated nitric acid).

Vanadium
(Ferrite former) gets alloyed in small amounts to the hardenable martensitic chromium steels for carbide forming in order to make the steel insusceptible to overheating. In addition, it enhances thermal resistance.

Titanium
(Ferrite former) in ferritic and austenitic steels bonds carbon to titanium carbide and nitrogen to titanium titride and thus makes the steel insusceptible to intercrystalline corrosion.

Niobium
(Ferrite former) also bonds the carbon in chromium steels and chromium/nickel steels and thus prevents intercrystalline corrosion.

Nickel
(Austenite former) dependent on the level of alloy content extends the austenite’s condition range up to below room temperature and along with chromium is the most important alloy element for stainless austenitic steels. Nickel improves the corrosion resistance and improves in particular resistance against stress corrosion.

Manganese
(Austenite former) has no identifiable influence on corrosion resistance. In austenitic chromium/nickel steels it impedes the transmutation of the austenite into alpha-martensite under mechanical working or low-temperature stress.

Carbon
(Austenite former) is the main accompanying element of all steels. It greatly extends the austenite area. Its effectiveness at low levels of content is c. 30 times greater than that of nickel. For reasons of corrosion chemistry the level of carbon content in most stainless steels is kept very low.

Nitrogen
(Austenite former) has a similar effect to carbon. The solubility of nitrogen increases in chromium steels and chromium/nickel steels as the chromium content goes up. In austenitic steels nitrogen increases the stability of the austenite and at the same time enhances strength qualities without reducing levels of ductility.

Copper
Gets alloyed to austenitic steels in special cases to improve corrosion resistance and/or to improve cold heating characteristics.

Sulphur
Contributes to improving machining (free-cutting stainless steels, e.g. 1.4305). However, the increased level of sulphur content leads to an impairment of the corrosion resistance, which needs to be taken into account when selecting materials for certain exposure conditions. The sulphur also makes the material less easy to weld.

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