Crevice corrosion, how does it occur and how to stop it?

Inconel and hastelloy alloys have suitable performance against crevice conditions for example hastelloy C276.

1. Crevice corrosion, how does it occur and how to stop it?

Inconel and Hastelloy alloys are commonly used in a wide range of applications where corrosive
conditions occur. Chromium in Inconel and Hastelloy and molybdenum in Hastelloy have a
considerable contribution in offering their corrosion resistance properties. Molybdenum in Hastelloy
enhances the nature of surface from one comprising a more susceptible nickel oxide to the material
with a more corrosion resistance film comprising majorly chromium oxide. The presence of
chromium oxide decreases the crevice and enhances the resistance.
How does crevice occur?
Crevice is usually developed by contact between a material with other. The other material could be a
component like a connection for example a fastener made from the same or different material.
Corrosion due to crevice can be caused by exposure to the conditions or by contacting water, the
other surface beyond the crevice can drain and dry. Crevice corrosion occurs through same
mechanism as pitting. Although as with other types of corrosion, various factors influence the
corrosion rate such as environments, concentration and alloy’s composition.
Considering crevice corrosion is essential when choosing a material particularly where the conditions
are vigorous. Various alloys for example stainless steels, Inconel and Hastelloy grades are widely
used in vigorously corrosive conditions like seawater or high temperature conditions. However these
alloys have supreme corrosion resistance in aggressive corrosion conditions although there is some

2. sensitivity to pitting and crevice, the extent of corrosion and failure are based on concentration of
elements and vigorousness of corrosive conditions.
Temperature has a crucial role in crevice corrosion. As temperature increases, crevice corrosion
initiates and then propagates and metal loss speeds up. Nickel alloys are not much used in corrosive
conditions because they readily lose their surface film and experience crevice and pitting attack. So
addition of elements to pure nickel is required for use in the corrosive media.
Crevice corrosion of nickel alloys and stainless steel occurs when their surface film damages at local
points on the surface exposed to the corrosive conditions at which anodic dissolution occurs
although the most of the part of the surface remains passive. It is required to secure the metals or
alloys from the corrosion by using corrosion inhibitors.
Surface layer break down and how to stop it?
In the chloride media, when the concentration of chloride ions is low, the rate of passive layer
development far exceeds than the film cracking. Although the surface layer can be anticipated to be
less sturdy at the grain boundaries as compare to those over the grain interiors. So, when the
concentration of chloride ions increases the film breaks down. With increase in chloride
concentration, the breakdown of surface film is faster than its development. At this stage, any
weakness in the surface layer resulting from grain boundaries of the alloy and the passive layer
damage seems controlled by the presence of chloride ions. The process of layer breakdown and
redevelopment of the film continues as long as the critical potential reaches where the surface layer
damages completely and the crevice corrosion occurs. When the chloride concentration is
maximum, the whole surface of the metal damages. So, in order to prevent this type of condition, it
is essential to use alloys that are resistant to crevice corrosion for example nickel based super alloys:
Hastelloy alloys.