Pitting and its closely related form, crevice corrosion, cause significant problems across industry, yet don’t receive the attention they deserve. Maintenance professionals often focus on other types of corrosion, such as stress-corrosion cracking (SCC) and microbiological-influenced corrosion (MIC). But because pitting and crevice corrosion can be just as damaging, it’s important to understand what causes these lower-profile forms of attack, where to find them and what corrective actions to apply.
Materials most susceptible to pitting and crevice corrosion are metals whose corrosion-resistance is based on their capacity to form protective oxide (passive) films. A passive film will protect the substrate metal as long as it remains intact. When the film breaks down and the material cannot repair itself, corrosion occurs. Film breakdown at discrete locations results in localized corrosion, which is most likely to affect stainless steels, nickel, copper and aluminum alloys. Carbon steels are also susceptible to localized corrosion, but less so, due to their weaker and essentially non-repairable films. Film breakdown on carbon steel is usually widespread, which leads to general rather than discrete corrosion.
Fig. 1. This cross-section illustration shows the internal attack by one type of crevice corrosion on two overlapping plates that aren’t sealed at the opening (both plates don’t have to be metallic).
Fig. 2. Pitting is most often found on the lower, horizontal areas in equipment where wetting is likely (i.e. as illustrated here in the 6 o’clock position on a horizontal tank, pressure vessel or pipe).
CORRECTION: In the original print and digital editions of this article, the captions for Fig. 1 and Fig. 2 were inadvertently reveresed. Please note this online version contains the corrected captions.
Pitting and crevice corrosion (shown in Figs. 1 and 2) have several common characteristics: Both are localized forms of attack, for example, where corrosion is concentrated and therefore more dangerous than general corrosion. Localized corrosion speeds up metal penetration, and the irregular surfaces it generates can apply stress that leads to other forms of corrosion, such as SCC. Also, corrosion fatigue can start at the bottom of pits or in the pit-like features created inside crevices. Because areas typically prone to pitting and crevice corrosion are relatively small, they can easily go undetected with traditional non-destructive examination techniques.
All corrosion in aqueous corrosive media is electrochemical, meaning that it has both electrical and chemical aspects. Two types of reactions take place:
1. Anodic reaction occurs through chemical oxidation when metal atoms with no electrical charge lose electrons and form positively charged ions of that metal.
2. Cathodic reaction occurs when the lost electrons travel through the metal to a site where they then react with chemical species in the corrosive solution (the electrolyte).
Anodic and cathodic reactions occur at the same rate, so slowing or accelerating either has that same effect on the other. This explains why certain actions decrease or increase the rate of corrosion. Both pitting and crevice corrosion are said to be autocatalytic in that the overall electrochemical action is self-accelerating.
Pitting occurs on a freely exposed metal surface where the anodic oxidation reaction is confined inside each pit and the matching cathodic reduction reactions occur on the immediate surrounding area. The surface condition of a metal exposed to an electrolyte is an important factor in predicting the probability of pitting. Corrosion is more likely if the surface is rough or has scratches or grooves.
In a crevice attack, the anodic reaction occurs inside a partially closed-off area, while a cathodic reaction occurs just outside that area. For a partially closed region to act as a crevice it must be open enough for the electrolyte to wet it inside but not open enough to allow free circulation in and out.
Thus, the size of a crevice site is critical to the probability of attack occurring. Attack at a crevice will initiate and progress before pitting starts on the free surface of the same metallic material if there is exposure to the same process conditions.
Finding pit and crevice corrosion
Corrosion rates are typically accelerated by high service temperatures, high concentrations of certain ions in electrolytes (especially chloride ions), the presence of oxygen or certain oxidizing ions such as ferric or cupric ions, and service situations that allow stagnant or very low-velocity electrolyte exposure to the metal. On stainless steels, attack is promoted by low-pH electrolytes. Such corrosion-promoting conditions may not be constant, but often occur because of process upsets.
Aqueous corrosion requires a liquid that can form ions. Residual water and other liquids that ionize and are not fully drained from equipment during shutdowns present the potential for pitting, crevice and all forms of aqueous corrosion. Attention to details of construction during initial design can minimize this issue. But inspectors should be aware that it’s not always possible to eliminate all areas that cannot be completely drained. At minimum, they should determine where such areas exist in their operation.
Pitting sites are most often found on the lower, horizontal areas in equipment where wetting is likely. Heat-affected zones (HAZ) of welds are often pitting sites, especially if the weld heat has sensitized the alloy to intergranular attack and the local electrolyte is aggressive to the particular alloy. Rough surfaces on weld beads are also susceptible to pitting, while weld splatter left to remain on equipment can invite crevice attack.
Creviced sites must also be wetted by an electrolyte, but an attack can occur on vertical and horizontal surfaces. Deposits of dirt and other debris that are wetted in service provide crevice sites that can easily be overlooked during inspections. Metal surfaces under these deposits generally give the appearance of pitting, as do the inside edges of most crevices.
Specific crevice examples are plentiful because it is difficult to fabricate something without creating at least some crevices. They include the unsealed interfacial space between lap joints (only one of the lapped materials needs to be metallic); the unsealed interfacial edges of “saddles” and horizontal tanks that rest in them; butt- or fillet-welded structural members joined by skip (tack) welds rather than continuous weld beads; the gasket surface in a pipe flange, especially if the gasket material is absorbent; and deposits of soil or debris on a wetted metallic surface.
Crevice corrosion is sometimes observed at the interface between water and air (or other gas) in storage tanks and pressure vessels. The area just above this line is often freely exposed to oxygen while the area just below the line will see much less. This arrangement encourages a cathodic oxygen reduction reaction above the line and anodic metal oxidation below it.
Taking corrective measures
The four traditional corrosion control measures call for use of the following:
Any one of the above measures can provide resistance to both pitting and crevice corrosion—and at least one, if not more, should be in place in any manufacturing environment. In some areas, specialists may be needed to recommend choices, particularly when applying corrosion inhibitors for metal alloys that are most susceptible to pitting and crevice attack. Interestingly, use of an inhibitor concentration that is too low can cause faster corrosion rates than would be found on such alloys with no inhibitor present.
When seeking more-resistant alloys for use in repair or replacement of the popular austenite stainless steels, it is important to recognize the role of particular elements in providing improved service. Alloys with more chromium, nitrogen and especially molybdenum are most effective in increasing resistance where high concentrations of chloride ions are present. Of course, the initial cost of the replacement material will be greater as the percentages of these elements increase, but the life-cycle cost versus use of lesser alloys may well be less. Other options to consider include nickel-based alloys and titanium alloys, depending on service conditions.
If resources allow, another corrosive-prevention measure can be to redesign equipment during repair or replacement work, and introduce measures that reduce or eliminate corrosion opportunities. This might include changes that would eliminate crevices by sealing them shut or increasing the opening size; eliminate “dead legs” and other piping and equipment features where the process liquid is stagnant or has low velocity; or allow for easier cleaning of deposits that may collect in service and form crevices. Other changes might include the introduction of slopes to piping or equipment to ensure that full drainage occurs during prolonged out-of-service periods.MT
FYI: Jerry Davis will discuss the topic of “Identifying and Controlling Corrosion,” in a regular MARTS 2013 Conference presentation, on Wednesday, May 1. To learn more and/or register, visit www.MARTSconference.com.