TECHNICAL INSPECTIONS OF CARGO TANKS AND SCRUBBERSTECHNICAL INSPECTIONS OF CARGO TANKS AND SCRUBBERS

Pitting Corrosion

Pitting corrosion most often takes place at points where the passive layer might be weakened, for example by slag inclusions, a damaged surface or imperfections in the passive layer.

Pittings may take the shape of small holes, 1-2 mm in depth and diameter to begin with. At times they are almost hidden below the surface of the steel with an "entrance hole" of only a few tenths of a mm. with a cavity of 1-2 mm below. They are therefore sometimes very difficult to detect, but might cause severe damage if left unattended.

Pitting may develop in a generally corrosive surrounding, especially when the surface is disturbed or when some extra chemical aggressive agent is present such as:

  • Chlorides (seawater, "salt") Contents above 200 ppm are generally dangerous when together with some other corrosive agent such as phosphoric acid. Fluorides have a similar effect.
  • Particles of iron or other materials (fremdkorper) on the surface
  • Craters or pores in weld deposits
  • Surface defects such as micro cracks and rough surfaces
  • Lack of oxygen renders formation of chromium oxide difficult
  • High temperatures above 400oC corrosion rates increase rapidly
stainless-steel-surface-inspectionstainless-steel-surface-inspection

Pitting Corrosion - Brief Facts

  • Pitting most often takes place at points where the passive layer might be weakened, for example by slag inclusions, a damaged surface or imperfections in the passive layer

  • Pitting corrosion takes place in neutral or acidic solutions containing primarily chlorides (Cl-), for example in neutral seawater

  • Factors influencing Pitting: Cl- content, pH value, temperature, presence of oxidizing agent

  • Starting Pitting (see pic) can be detected by white dots on the surface and need to be properly cleaned before further deterioration of the surface into actual pits

  • Pitting corrosion can cause severe damage if left unattended

STAINLESS STEEL SURFACE BUFFING - GRINDING - POLISHING - WELDING
CLEANING OPERATIONS - PICKLING AND PASSIVATION

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