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.
Two factors are important in the initiation of active crevice corrosion: the chemical composition of the electrolyte in the crevice and the potential drop into the crevice.
Some of the phenomena occurring within the crevice may be somewhat reminiscent of galvanic corrosion.
two connected metals + single environment
one metal part + two connected environments
Deposits of sand, dust, scale and corrosion products can all create zones where the liquid can only be renewed with great difficulty.
Crevice corrosion is encountered particularly in metals and alloys which owe their resistance to the stability of a passive film, since these films are unstable in the presence of high concentrations of Chlorine- and Hydrogen ions.
Combating Crevice Corrosion
1, The simplest method for preventing crevice corrosion is reducing crevices in the design of the Structure.
2, When it is not possible to get rid of crevices, improving drainage and sealing of edges or keeping crevices as open as possible.
4, preventing entrance of moisture is the best protective action .
5, A protection method called hot wax dip is commonly used in automotive industry.
6, In this method faying surfaces that will make crevices are usually painted before assembly.
7, In aerospace industry sealing the faying surfaces with a polysulfide is known to be an effective method for preventing crevice corrosion.
8, Cathodic protection could be an effective method against crevice corrosion, but anodic protection is often improper.
9, Another common protection method is using alloys which are less vulnerable to crevice corrosion.
10, Addition of inhibiting substances to bulk solution is also a protection method. Application of passivating compounds such as chromate and nitrate is well practiced to prevent crevice corrosion.
11, Overlaying susceptible areas with an alloy which is more resistant to crevice corrosion is another protective measure.
This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating.
Lacquers (hard varnish for wood and metal) and "quick-dry" paints are most susceptible to the problem.
Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating.
Preventions for filiform CorrosionThe use of Lacquers (hard varnish for wood and metal) and "quick-dry" paints should be avoided unless absence of an adverse effect has been proven by field experience.
Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion.
Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality.