While many forms of corrosion are obvious with large areas of deterioration and discoloration, crevice corrosion can start with a very small surface patch and spread quickly out of sight within the metal, underneath insulation, or inside tanks and vessels, leading to component or structural failure with little to no warning.
Fortunately, the causes of crevice corrosion are well known.
With knowledge of what encourages this form of corrosion and the steps you can take to prevent it offered in this guide, minimizing risk is easy.
What is Crevice Corrosion?
Crevice corrosion is a localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces when exposed to a stagnant electrolyte.
This can occur between two metals or a metal and nonmetal material — even if both materials would otherwise resist corrosion without the presence of a gap.
Typically, the electrolyte won’t pose a problem at first. However, as it remains in the crevice, oxygen levels will deplete.
This then triggers a surplus of positive metal ions within the crevice, turning the interior of the crevice into an anodic node.
The surrounding solution will then attempt to balance this situation by sending negative ions into the crevice (often in the form of chloride ions) further enhancing acid creation and attacking the metal.
Common areas subject to crevice corrosion include:
- Bolted and riveted connections
- Under washers, gaskets, and clamps
- Under insulation
- At lap joints
- In fastener threads
If this all sounds a little complicated, just remember that when electrolyte can settle in a small area between a piece of metal and another surface (metal or non-metal), the risk for crevice corrosion increases.
This simple description gives you multiple ways to reduce the risk of corrosion occurring.
Preventing Crevice Corrosion
As the definition of crevice corrosion describes, there are two primary methods of reducing crevice corrosion risks:
- Eliminate small gaps which might trap electrolyte and lead to stagnation
- Keep electrolyte flowing freely or at high enough rates to prevent stagnation
However, a range of factors can influence both overall risk levels and the severity of the corrosion’s progression once underway:
- Crevice types: Metal to metal and metal to nonmetal crevices will typically behave differently.
- Physical crevice characteristics: Everything from gap size to the depth and surface texture in and around the crevice can influence corrosion risk and progression.
- Metal composition: Alloys will offer different levels of crevice corrosion resistance. In many cases, the same alloys which exhibit strong pitting corrosion resistance also offer improved crevice corrosion resistance.
- The use environment: Piping systems with ample room for electrolytes to circulate and with higher flow rates tend to resist crevice corrosion better. This is because there are fewer pockets of unmoving electrolyte and even when electrolyte might be trapped, higher flow rates help to exchange trapped electrolyte with fresh quick enough to avoid the initiation of corrosion.
- The nature of the electrolyte solution: Solution temperature, pH levels, oxygen levels, halide or chloride ions, and other traits can all influence not only corrosion risks but how aggressively the corrosion will spread should an attack commence.
Exact steps to reduce or eliminate crevice corrosion risk will vary based upon your piping system design, operating environment, and intended usage.
However, the following are good starting points that are likely to impact a range of piping systems and use cases:
- Use butt weld joints instead of bolted or threaded joints for new equipment
- Use continuous welding or soldering to eliminate crevices in lap joints
- Ensure complete vessel drainage where possible and design any non-draining areas to allow sufficient solution flow to prevent stagnation
- Use solid, non-absorbent gaskets where possible
- Use alloys rated higher by ASTM G48 standards for increased resistance to crevice corrosion
- Crevice corrosion requires 2 conditions: a gap between a metal surface and another metal or nonmetal surface and the presence of a stagnant electrolyte.
- Crevice corrosion is not always immediately apparent without close visual inspection.
- Eliminating gaps, such as those found in welds and joints, can help to reduce crevice corrosion risk.
- Ensuring complete drainage of vessels and removing electrolytes from surfaces can further reduce risk.
- Utilizing solid, non-absorbent gaskets can reduce risk at connections and equipment.
- Materials with a strong resistance to pitting corrosion often exhibit high crevice corrosion resistance as well.
Unified Alloys has the experience and materials selection to help industries across Canada and North America with projects big and small. As a leading provider of stainless steel products for more than 4 decades, we have an intricate understanding of the typical situations and operating environments which might lead to crevice corrosion. Call today and discuss your needs with one of our expert sales analysts to find a solution that will ensure optimal performance for your intended usage.
- Sandvik: Crevice Corrosion
- WebCorr: Different Types of Corrosion – Recognition, Mechanisms & Prevention
- Corrosion Doctors: Crevice Corrosion
- British Stainless Steel Association: Principles and prevention of crevice corrosion
- Corrosionpedia: What’s the Inside Scoop on Crevice Corrosion?
- NACE International: Crevice Corrosion
- Nitty Gritty: Crevice Corrosion
- Facility Executive: Crevice Corrosion, And What To Do About It
- Wikipedia: Crevice Corrosion
- ThoughtCo: What is Corrosion?