Corrosion related surface defects for stainless steel

Corrosion related surface defects for stainless steel

Iron Contamination

Iron contamination is probably the most significant corrosion related surface defect for stainless steel. The carbon steel particles can be easily picked up through steel lay yard, cutting, forming rolls, steel wire brushing, sand blasting, grinding. The embedded iron particles can form local galvanic cells and lead to pitting of stainless steel in moist atmosphere.

Heat Tint

Heat tint is developed in heat treatment process or welding process. The heat will produce heavy oxide films. For stainless steel application, most of scale is developed in the HAZ of weldment. The colour of oxide films ranges from light brown to blue, mainly pending on the film thickness. Heat tint is a source of pitting and cracking initialisation in corrosive environment, e.g. desalination process.

Weld flux residue

Weld flux residue is a slag produced by welding process. Mechanical cleaning process, such as wire brushing, disk grinding and wheel grinding, will always leave small flux particles. These will cause crevice corrosion near weldment. This is particular serious for complicate welding shapes. The pickling process is necessary for complete removal of flux residue.

Surface scratch

Deep surface scratch is also an initialisation source of corrosion for stainless steel. Normally,  a deep scratch is easy access of contaminant e.g. water and chloride. However the oxygen, which is required for passivating the stainless steel, is not readily accessible. This will lead to the breakdown of passivation film and initialising the pitting corrosion.

Inspection

ASTM A 380 “Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems”  is a good reference on stainless steel cleanness inspection practice.

• For greasy or other organic contamination:

Water-break test is an easy and effective in detecting residual organic matters. A sheet of water on stainless steel surface will break around oil/grease.

• For iron contamination

Water testing: leave the water on the stainless steel surface to observe any stain within a few hours. A rust stain is an indicator of iron contamination.

Copper sulphate testing: copper sulphate solution on the stainless steel surface to observe any copper observed. This is more sensitive to detect embedded iron on stainless steel suface

Ferroxyl testing: any trace of iron ion will display blue in stainless steel surface.

 

Digestion  from  Arthur H. Tuthill and Richard E . Avery “Specifying Stainless Steel Surface Treatments“