Common defects of stainless steel

Paradoxically, the scale of “resistance” hidden in the name is determined by the surface of the stainless steel, which should be clean, smooth and flawless. This is an essential condition for corrosion resistance.

In fact, stainless steel is only protected against corrosion by a thin, impermeable surface layer – the passive layer – which consists mainly of chromium oxide. The oxygen content in the atmosphere or in oxygenated water solutions is generally sufficient for the formation and maintenance of this passive layer. Unfortunately, surface defects and imperfections from manufacturing operations can disrupt the re-filming process and limit resistance to several types of localized corrosion.

This means that a cleaning process in the final stage of stainless steel treatment is often necessary to restore acceptable surface quality in terms of both cleanliness and corrosion.

Below we present the alloy components, ie CHROME, NICKEL, etc., typical for the composition of stainless steels.

Common defects include:

Welding defects

Undercuts, welding spatter, incomplete fusion, or arc strikes are just a few common examples of welding defects. Such defects not only affect the resistance and local corrosion, but also the mechanical properties of the material. As a rule, in such cases, the defect is removed by grinding or repair welding.

Flash color and oxide scale

High temperature oxidation – caused by processes such as heat treatment or welding – creates an oxide layer with inferior protective properties compared to the original passive layer. Additionally, an associated chromium depletion of the metal occurs immediately under the oxide. In order to restore corrosion resistance, it is necessary to remove the depleted zone that is under the tarnish formed during welding. Usually it is removed together with the bloom.

Surface roughness

Excessive grinding, shot blasting, or uneven welding seams can make the surface rough and therefore promote the build-up of deposits. At the same time, the risk of both product contamination and corrosion increases. Often, high tensile stresses occur during intensive grinding, which increase the risk of pitting corrosion and stress corrosion cracking. Despite the existence of the indexes of the maximum permissible surface roughness, methods leading to the formation of surface roughness should generally be avoided in the production process.

Iron contamination

Iron particles can arise as a result of mechanical treatment from:

• cold forming or cutting equipment
• sanding belts and discs contaminated with low-alloyed materials
• sand or shot blasting equipment

Additionally, when materials are handled during multi-stage production, transportation or simply by settling of dust containing iron particles. These particles corrode in humid air and damage the passive layer (in the case of larger particles, it can even lead to cracks).

Organic pollution

Greases, oils, paints, glue residues or even footprints are common organic contaminants that can cause crevice corrosion in susceptible environments. In addition, they can contaminate products that come into contact with the equipment and prevent effective surface etching. This type of contamination should be removed with a suitable pre-cleaner or degreaser. In uncomplicated cases, a pressurized water jet can be used.

Nevertheless, there are various methods, both chemical and mechanical, whether used alone or in combination, that can be used to remove the above defects.

Some of the methods used:

• Mechanical methods
  • grinding
  • sanding
  • shot blasting
  • brushing
• Chemical methods
  • digestion
  • electrolytic polishing
  • passivation and decontamination

The choice of method or methods depends on many factors, however, it should not be forgotten that the choice should be made on the basis of both the defects that have occurred, but also with regard to the requirements for surface quality and subsequent application.