The structural integrity of metal structures depends heavily on the strength of the connections between individual members. These connections, most times, come in the form of bolts and washers, rivets, screws and welding.
Although two metal components can appear to be firmly fastened together, small gaps can exist between the connected pieces. Over time, corrosion can form at the interface between the connected metals that can significantly weaken the connection’s load carrying capacity. (Learn why in the article Effect of Corrosion on a Material's Tensile Strength and Ductility.) This form of corrosion is called crevice corrosion because it is formed at the crevice between two joining metal surfaces.
In this article, we shall look at the phenomenon called crevice corrosion and explain the factors that cause it, describe the underlying mechanisms involved and outline the factors that affect its formation.
What is Crevice Corrosion?
Crevice corrosion is defined as an intensely localized corrosion on a metal surface that frequently occurs at, or directly adjacent to, a gap or crevice between the two connected surfaces. The crevice can be between metal to metal or metal to non-metal contact areas that are sometimes called faying surfaces. (For an example of metal to non-metal corrosion, see Galvanic Corrosion of Metals Connected to Carbon Fiber Reinforced Polymers.) Typically, outside of the crevice the metal substrate remains uncorroded.
For crevice corrosion to occur, an electrolyte solution (such as water) must be present in the gap between the connected surfaces. In addition, this solution must also ideally be stagnant (i.e., there must be no significant movement of the electrolyte in and out of the crevice). These gaps are commonly found under gaskets, washers, bolt heads, insulation material, spliced joints, fasteners and clamps.
Narrow gaps can also be formed by deposition of dirt, mud, biofouling and other deposits. For example, a deposited object at the bottom of a water tank can form tiny crevices (depending on the object’s shape) around its perimeter.
The Mechanisms Involved in Crevice Corrosion
This type of corrosion, though simple in appearance, involves a system of surprisingly complex chemical reactions. These reactions initiate corrosion by changing the local chemistry of the stagnated electrolyte within the crevice. The corrosion mechanism can be broken down into four stages.
- Oxygen depletion in the crevice – Initially, the soluble oxygen contents inside and outside the crevice are equivalent, and anodic/cathodic processes occur equally on all the surfaces of the metal. However, because oxygen diffusion is restricted in the crevice, cathodic reactions cannot be sustained, thus giving rise to oxygen depletion within the crevice. The uneven level of oxygen between the internal (crevice) and external bulk areas results in a differential aeration cell.
- Increase of acidity in the crevice – With the oxygen depleted inside the crevice, oxygen now diffuses from the surrounding electrolyte on the surface of the metal to form a surplus of positive metal ions inside the crevice. The abundance of positive ions makes the micro-environmental condition within the crevice anodic in nature. This anodic imbalance sets up a potential difference that causes the migration of negative ions (usually in the form of chloride ions) from the solution into the crevice in an attempt to balance the surplus positive charge. The chloride ions act aggressively towards the metal and also react with the positive metal ions to produce acids that make the solution within the crevice highly corrosive. The process is autocatalytic because the resulting acid can also combine with the already existing chloride ions to encourage further corrosion.
- Passive layer breakdown – If the metal consists of a passive layer, the corrosive nature of the electrolyte within the crevice will eventually cause this layer to break down, leaving the metal substrate defenseless against corrosion attack. (Discover the role of passive layers that protect stainless steel in Using Pickling and Passivation Chemical Treatments to Prevent Corrosion.)
- Propagation of the crevice corrosion – Finally, with the passive layer broken down, the corrosion process can continue unhindered as the metal substrate is now fully exposed to the acidic and corrosive solution within the crevice.
Factors that Affect Crevice Corrosion
Similar to other types of corrosion, the rate and intensity of crevice corrosion are dependent on several factors. Some of these include:
- Bulk solution composition – Since the corrosion attack is initiated by the difference in oxygen concentration between the solution in the crevice and the bulk solution, the chemical makeup (such as soluble oxygen content) of the bulk solution will have a direct effect on the aggressiveness of the corrosion.
- Nature of solution in the crevice – Due to chemical changes the properties of the solution in the crevice determines its aggressiveness towards the metal. Some of these properties include pH, amount of chloride ions, temperature, oxygen content, etc.
- Stagnancy of the solution in the crevice – The more stagnant the solution in the crevice, the faster the influx of chloride ions and the faster the rate of the corrosion attack.
- Alloy composition – Metal alloys that are more resistant to corrosion attack will corrode slower than highly reactive metals, thus affecting the overall rate of the corrosion process. The crevice corrosion index is a way to estimate a metal's susceptibility.
- Crevice type – The size and shape of the crevice are an important factor in this type of corrosion. Corrosion is more aggressive when the gap is big enough to allow the solution to seep in but small enough to maintain stagnancy of the solution in the crevice. A gap that is too big or too small would not form conditions that are ideal for corrosion to occur.
Crevice corrosion can be very aggressive if left untreated. The most benign atmospheric environments, given the right conditions, can facilitate this type of attack with devastating effects.
Pitting, filiform corrosion, intergranular attack and chloride stress corrosion cracking (CSCC) are just some of the forms that crevice corrosion can take. It is imperative that engineers and contractors take measures to avoid creating situations where this type of corrosion can appear.