COMMON DEFECTS OF STAINLESS STEEL

The surface of stainless steel which should be clean, smooth and flawless determines its “resistance" level. This significantly conditions the corrosion resistance.

 In practice, stainless steel is protected against corrosion only by a thin, impermeable surface layer, that is, a passive layer mainly composed of chromium oxide. Oxygen content in the atmosphere or aerated aqueous solutions is usually sufficient to form and maintain this passive layer. Unfortunately, flaws and defects on the surfaces created during manufacturing operation can disturb the process of reforming this layer and reduce the metal resistance to a few types of the local corrosion. This means that the cleaning process in the final phase of stainless steel treatment is often required to restore the acceptable quality level of the surface regarding both the cleanliness and the corrosion.

Welding defects

Undercutting, metal spattering, incomplete penetration or arc ignition are just typical examples of welding defects. Such defects influence the corrosion resistance and the formation of local corrosion as well as affect the mechanical properties of the material. In these cases, the defect is usually eliminated by grinding it off or conducting repair welding.

Heat tint and oxides scale

Oxidation at high temperatures – caused by such processes as heat treatment or welding – forms the oxide layer of worse protective properties than the properties of the original passive layer. Moreover, the related chromium loss in the metal, directly below the oxide layer, occurs. The elimination of the impoverished zone placed under the heat formed during the welding process is necessary for restoring the metal resistance. It is removed along with the heat tint using the pickling pastes (Turbo Z) or gels (PROCAP).

 
Surface roughness

Excessive grinding, blast cleaning or uneven welding seams can cause roughness of the surface, which consequently foster the accumulation of deposits. At the same time, there is a greater risk of product contamination and corrosion. During the intensive grinding, high tensile stresses can occur which increase the risk of pitting corrosion and stress corrosion cracking. Despite the indexes of maximum allowable surface roughness, the methods resulting in surface roughness should be generally avoided in the production process.

 
Iron impurities

Iron particles can be formed during the mechanical treatment of:

• equipment for cold rolling or cutting
• abrasive strips and disks contaminated with low-alloyed materials
• equipment for sand blasting and blast cleaning

Additionally, these impurities can be caused by conveying the materials during a multi--stage production process, transporting them or simply by the sedimentation of dust with iron particles These particles corrode in the wet air, destroy the passive layer (cracks can appear in case of larger particles).

Organic fouling

Lubricants, oils, paints, glue residues or even shoe prints are examples of typical organic fouling which can result in the crevice corrosion in vulnerable environments. Moreover, they can also contaminate products being in contact with the particular equipment or preclude effective surface pickling. This type of fouling should be removed using a suitable agent for preliminary cleaning or degreasing (a range of PROSOLV products is recommended). In the simple cases, a stream of hot water under pressure or degreasing with ARDROX sprays can be applied.