Crevice corrosion is a corrosion occurring in spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles. Components subject to fouling The following lists examples of components that may be subject of fouling and the direct effects of fouling heat exchanger surfaces

Mechanism

Crevices can develop a local chemistry which is very different from that of the bulk fluid. For example, in boilers, concentration of non-volatile impurities may occur in crevices near heat-transfer surfaces because of the continuous water vaporization. "Concentration factors" of many millions are not uncommon for common water impurities like sodium, sulfate or chloride.

Crevice corrosion generally occurs due to differential electrolyte chemistry inside and outside the crevice: a single metal part undergoing corrosion is submerged in two different environments. This situation is somewhat reminiscent of galvanic corrosion:

galvanic corrosion

two connected metals + single environment

crevice corrosion

one metal part + two connected environments

The mechanism of crevice corrosion can be (but not always is) similar to that of pitting corrosion. Galvanic corrosion is an Electrochemical process in which one Metal Corrodes preferentially when in electrical contact with a different type of metal Pitting corrosion, or pitting, is a form of extremely localized Corrosion that leads to the creation of small holes in the metal However, there are sufficient differences to warrant a separate treatment. For example, in crevice corrosion, one has to consider the geometry of the crevice and the nature of the concentration process leading to the development of the differential local chemistry. The extreme and often unexpected local chemistry conditions inside the crevice need to be considered. Galvanic effects can play a role in crevice degradation.

Filiform corrosion is a type of crevice corrosion that may occur on an aluminium surface underneath an organic coating.

Stress corrosion cracking

The Silver Bridge upon completion in 1928

The collapsed Silver Bridge, as seen from the Ohio side

A common form of crevice failure occurs due to stress corrosion cracking, where a crack or cracks develop from the base of the crevice where the stress concentration is greatest. Fracture mechanics --> Stress corrosion cracking ( SCC) is the unexpected sudden failure of normally Ductile metals or tough Thermoplastics A stress concentration (often called stress raisers or stress risers) is a location in an object where stress is concentrated This was the root cause of the fall of the Silver Bridge in 1967 in West Virginia, where a single critical crack only about 3 mm long suddenly grew and fractured a tie bar joint. The Silver Bridge was an Eyebar Chain Suspension bridge built in 1928 and was named for the color of its Aluminum paint West Virginia ( is a state in the Appalachian Upland South, and Mid-Atlantic regions of the United States, bordered by The rest of the bridge fell in less than a minute. The eyebars in the Silver Bridge were not redundant, as links were composed of only two bars each, of high strength steel (more than twice as strong as common mild steel), rather than a thick stack of thinner bars of modest material strength "combed" together as is usual for redundancy. With only two bars, the failure of one could impose excessive loading on the second, causing total failure—unlikely if more bars are used. While a low-redundancy chain can be engineered to the design requirements, the safety is completely dependent upon correct, high quality manufacturing and assembly.

Significance

The susceptibility to crevice corrosion varies widely from one material-environment system to another. In general, crevice corrosion is of greatest concern for materials which are normally passive metals, like stainless steel or aluminum. Crevice corrosion tends to be of greatest significance to components built of highly corrosion-resistant superalloys and operating with the purest-available water chemistry. A superalloy, or high-performance alloy, is an Alloy that exhibits excellent mechanical strength and creep resistance at high temperatures good surface For example, steam generators in nuclear power plants degrade largely by crevice corrosion. This is an article about nuclear power plant equipment For other uses see Steam generator.

Crevice corrosion is extremely dangerous because it is localized and can lead to component failure while the overall material loss is minimal. The initiation and progress of crevice corrosion can be difficult to detect.

External Links

• Crevice Corrosion at Pipe Supports: Causes and Solutions (paper)