Ammonia Stress Corrosion Cracking: Mechanism, Materials and Prevention

RUNNING HEAD: Corrosion
Corrosion

Corrosion 1
Contents
Introduction......................................................................................................................................2
Ammonia Stress Corrosion Cracking..............................................................................................2
Mechanism of Ammonia Stress Corrosion Cracking......................................................................3
Affected Materials and Equipment..................................................................................................5
Appearance/ Morphology/ Damage.................................................................................................6
Prevention/ Mitigation.....................................................................................................................8
Conclusion.......................................................................................................................................9
References......................................................................................................................................11

Corrosion 2
Introduction
Corrosion can be defined as a natural process with the help of which refined metal is converted
into more chemically- stable form such as its sulfide, hydroxide or oxide (Talbot & Talbot,
2018). It is the slow destruction of materials which are usually metals by electrochemical and / or
chemical reaction within their environment. Materials other than metals such as polymers or
ceramics also suffer from corrosion, however, the term degradation is more commonly used in
this context. The valuable properties of materials and structures are degraded as a result of
corrosion including the appearance, strength and permeability to gases and liquids. Passivation is
beneficial in the mitigation of corrosion damage. The hindrance of passivating film forming
capability can even lead to the corrosion of high- quality alloy. Right grade of material is
required to be selected for the particular environment in order to ensure the long- lasting
performance of such group of materials. If the passive film suffers from a breakdown as a result
of mechanical or chemical factors, the resultant modes of corrosion may include crevice
corrosion, pitting corrosion and stress corrosion cracking. The focus of this report is on ammonia
stress corrosion cracking.
Ammonia Stress Corrosion Cracking
Stress corrosion cracking (SCC) can be demarcated as the growth of crack formation in a
corrosive environment (Cunningham, Bottleberghe & Greene, 2017). The protective oxide layer
is sometimes destabilized by the corrosive environment under definite conditions without
causing general corrosion. The reformation of oxide is prevented after a crack due to
destabilization. Ductile metals that are subject to a tensile stress have chances of facing an
unexpected sudden failure specifically at elevated temperature. Stress corrosion cracking is
caused by the chemical environment which is only a little corrosive to metal (Cheng, 2013).
Brightness is noticed in metals with faces severe stress corrosion cracking while they are filled
with cracks that are microscopic. This is the most common factor as a result of which the stress
corrosion cracking goes undetected before its failure. Stresses can also be the outcome of crevice
loads because of stress concentration or can be instigated by residual stresses or type of
assembly.

Corrosion 3
Ammonia stress corrosive cracking is a form of SCC that takes place in brass tubes in cooling
water service which has been contaminated by ammonia as a result of biological growths or any
other contamination. The occurrence of this cracking can also be the consequence of intentional
addition of ammonia as a neutralizer to the process streams by someone who is not aware its
impact on the brass tubes. Brittle fracture is experienced on bending by the brass condenser tubes
on the presence of significant ammonia stress corrosion cracking (Raja & Shoji, 2011).
Carbon steel equipment are also affected by the ammonia stress corrosion cracking but unlike the
mechanism of cracking on brass which takes place in an aqueous solution, steal equipment
cracking takes place in an anhydrous ammonia. In other words, the presence of liquid ammonia
in oxygen can result in stress corrosion cracking in carbon steels (Shi, et. al., 2015). With the
increase in the yield strength of the plate material, local hardness of the welds and increased
strength of welds, the probable problem of stress corrosion cracking is also increased.
Vulnerability to this issue is also faced by the systems that are contaminated with oxygen/ air.
However, cupro- nickel alloys are not found to be vulnerable to ammonia stress corrosion
cracking (McDougal & Stevenson, 2005).
During the normal operations, the applied stress levels are not high as required to initiate
cracking.The usage of eddy current such as eddy current array or pulsed eddy current testing is
considered to be one of the best techniques for the purpose of inspection of ammonia stress
corrosion cracking in brass tubes.
With specific reference to 18Cr-8Ni steels, the characteristics of stress corrosion cracking
provides that susceptibility is found in alloy to transgranular stress corrosion cracking at the time
when there is possibility of a passive/ active transition or when a noble surface is produced by
the responses in alloy surface which may be metal or oxide, co- planar arrays of dislocations are
exhibited by the alloy or high work- hardening rate is exhibited by the alloy, specific
electrochemical reactions are there (which are considered to be the chloride’s frequent
penetration of passive films) (Kim, Kwon, Kim, Hwang & Kim, 2011).
Mechanism of Ammonia Stress Corrosion Cracking
The proposal of various mechanisms has been made for the purpose of explaining the synergistic
stress corrosion interactions the occurrence of which is found at the crack tip. Stress corrosion