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Prevention of crevice corrosion

Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material. Outside the gap or without the gap, both metals are resistant to corrosion. Crevice corrosion is not unique to stainless steels. It can occur in other alloys including those of aluminium, titanium and copper.

The damage is normally confined to one metal at localized area within or close to the joining surfaces.

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A stainless steel tube and tube sheet from a heat exchanger in a desalination plant suffered crevice corrosion due to the presence of crevice (gap) between the tube and tube sheet.
The mechanism of crevice corrosion


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Crevice corrosion (beneath a seal) on a stainless steel flange exposed to a chloride-rich medium.

It is a localized form of attack, where there is a breakdown of the surface passive layer, in crevices or on 'shielded' areas beneath surface deposits. Engineered or 'designed in' crevices can be set up at bolted and other joints, beneath flanges or between flanges and gaskets or other contact areas such as valve seats.

Crevice corrosion is initiated by a difference in concentration of some chemical constituents, usually oxygen, which set up an electrochemical concentration cell (differential aeration cell in the case of oxygen).

Outside of the crevice (the cathode), the oxygen content and the pH are higher - but chlorides are lower.
Chlorides concentrate inside the crevice (the anode), worsening the situation. The pH and the oxygen content are lower in the crevice than in the bulk water solution, just as they are inside a pit. Once a crevice has formed, the propagation mechanism for crevice corrosion is the same as for pitting corrosion.

John Sedriks assumes that there is some initial general corrosion in the passive state. The release of metal ions (M+) at the anode is balanced by a reaction at a nearby cathodic site, which involves using up oxygen from the surroundings. In a crevice oxygen can soon be used up and if the conditions are stagnant enough, replenishment of the oxygen to the crevice from the bulk solution is stifled. If this reaction continues outside the crevice it can support a corrosion cell where the metal ions continue to be liberated in the anodic crevice. This results in a build up of the positively charged metal ions in the crevice. The mechanism then moves to a stage that involves the familiar affects of chloride and pH on crevice (and pitting) corrosion.

The negatively charged chloride ion (Cl-) is very mobile and so if present in the bulk solution outside the crevice, easily migrates into the narrow crevice under the attraction of the positively charge metal ions (M+). The metal chloride (usually involving chromium) formed in the crevice then reacts with water to form hydrochloric acid. The build up of acid reduces the pH until a 'depassivation pH' is reached where the passive film is breached locally in the crevice so that the 'pitting' attack can then continue.
Once depassivated the attack can progress rapidly and is self sustaining, especially if there is a ready supply of chloride ions from the bulk solution.

Ferrous ions form ferric chloride and attack the stainless steel rapidly.

Prevention
  • Use welded butt joints instead of riveted or bolted joints in new equipment
  • Eliminate crevices in existing lap joints by continuous welding or soldering
  • Use solid, non-absorbent gaskets such as Teflon.
  • Use higher alloys (ASTM G48) for increased resistance to crevice corrosion
Alloy selection to avoid crevice corrosion

The stability of the passive layer on stainless steels is promoted by chromium and supported by nickel. Although the chromium metal ion supports the anodic reactions in crevice corrosion more than iron or nickel, alloys with increasing chromium are better in resisting crevice corrosion. For given chromium level, austenitic stainless steels seem to resist attack better then the lower nickel ferritic types. Molybdenum and nitrogen have a very marked affect on increasing resistance, molybdenum assisting by arresting the rate of attack once depassivation has occurred and rapid attack is usually the next stage.

As a general rule stainless steels such as the 6% molybdenum austenitics can be expected to give the best crevice corrosion attack resistance. As a guide some common stainless steels, rated in decreasing resistance to crevice corrosion, follows
  • 1.4547 (254SMO) (6% Mo austenitic)
  • 1.4462 (2205)
  • 1.4539 (904L)
  • 1.4401/1.4436 (316)
  • 1.4301 (304)
  • 1.4016 (430)
'The Crevice Corrosion Engineering Guide' provides a mathematical model of crevice corrosion to assist in the selection of the austenitic stainless steels above in various water conditions. This is available as floppy disc, reference D0003, from the NiDI web site.

Avoiding crevices in design

  • Keep junction points as wide open as possible.
  • Avoid 'designed in crevices' that can be formed in flanged or bolted joints. Both metal / metal or metal / non-metal contacts can result in attack sites and so the use of insulating gaskets will not prevent crevice corrosion. Good fit-up and adhesion of non-metal joints or gaskets is important to avoid crevices being formed between the two materials.
Avoiding crevices during fabrication

  • Crevices can be formed during fabrication at welded joints where full bead penetration has not been achieved. Full root penetration is essential with as rounded and smooth a bead as possible, with no undercut in the internal bead to parent metal area.
Avoiding crevices during operation

Build up of scale or settlement in tanks can result in crevice (shielding) corrosion. Steps to prevent this or introducing a routine cleaning / maintenance program, where some build up is unavoidable, is worth considering.