Atmospheric Corrosion
115
added to stabilize the protective surface film. Alloys containing in excess of
15% zinc are susceptible to de-alloying in acid or alkaline environments.
There is a type of corrosion in which brass dissolves as an alloy and the
copper constituent redeposits from solution onto the surface of the brass as
a metal in porous form. The zinc constituent may be deposited in place of an
insoluble compound or carried away from the brass as a soluble salt. The corrosion can take place uniformly or locally. Uniform corrosion is more likely
to take place in acidic environments, while local corrosion is more apt to
take place in alkaline, neutral, or slightly acidic environments. The addition
of tin or arsenic will inhibit this form of corrosion. Conditions of the environment that favor dezincification are high temperature, stagnant solutions
(especially of acid), and porous inorganic scale formation. Other factors that
stimulate the process are increasing zinc concentrations, and the presence of
both cuprous and chloride ions. As the de-alloying proceeds, a porous layer
of almost pure copper is left behind. This reaction layer is of poor mechanical strength. The dezincification process on copper-zinc alloys is therefore
very detrimental.
High-zinc brasses are also susceptible to season cracking, which is a form
of stress cracking. The term originates from early in the twentieth century
when cartridge shells made of 70% copper and 30% zinc were found to
crack over a period of time. It was later realized that ammonia from decaying organic matter in combination with residual stresses in the brass was
responsible for the cracking of these shells. This phenomenon was called
season cracking because the presence of high humidity during warm, moist
climates (or seasons) promoted the stress corrosion cracking.
Stress corrosion of brass commonly occurs when brass is subjected to an
applied or residual stress or while in contact with a trace of ammonia or
amine in the presence of moisture and oxygen. The risk of stress corrosion
cracking in brasses is greatest in industrial and urban atmospheres, characterized by high contents of sulfur dioxide and ammonia. The stress corrosion
susceptibility is markedly lower in marine atmospheres. The relative resistance to stress corrosion cracking of the brasses is as follows:
Low resistance:
•
Brasses containing more than 15% zinc
•
Brasses containing more than 15% zinc and small amounts of
•
lead, tin, or aluminum
Intermediate resistance:
•
Brasses containing less than 15% zinc
•
The admiralty and the naval brasses are resistant to de-alloying as a
result of the addition of tin. High-zinc brasses resist sulfides better than
low-zinc brasses.
115
added to stabilize the protective surface film. Alloys containing in excess of
15% zinc are susceptible to de-alloying in acid or alkaline environments.
There is a type of corrosion in which brass dissolves as an alloy and the
copper constituent redeposits from solution onto the surface of the brass as
a metal in porous form. The zinc constituent may be deposited in place of an
insoluble compound or carried away from the brass as a soluble salt. The corrosion can take place uniformly or locally. Uniform corrosion is more likely
to take place in acidic environments, while local corrosion is more apt to
take place in alkaline, neutral, or slightly acidic environments. The addition
of tin or arsenic will inhibit this form of corrosion. Conditions of the environment that favor dezincification are high temperature, stagnant solutions
(especially of acid), and porous inorganic scale formation. Other factors that
stimulate the process are increasing zinc concentrations, and the presence of
both cuprous and chloride ions. As the de-alloying proceeds, a porous layer
of almost pure copper is left behind. This reaction layer is of poor mechanical strength. The dezincification process on copper-zinc alloys is therefore
very detrimental.
High-zinc brasses are also susceptible to season cracking, which is a form
of stress cracking. The term originates from early in the twentieth century
when cartridge shells made of 70% copper and 30% zinc were found to
crack over a period of time. It was later realized that ammonia from decaying organic matter in combination with residual stresses in the brass was
responsible for the cracking of these shells. This phenomenon was called
season cracking because the presence of high humidity during warm, moist
climates (or seasons) promoted the stress corrosion cracking.
Stress corrosion of brass commonly occurs when brass is subjected to an
applied or residual stress or while in contact with a trace of ammonia or
amine in the presence of moisture and oxygen. The risk of stress corrosion
cracking in brasses is greatest in industrial and urban atmospheres, characterized by high contents of sulfur dioxide and ammonia. The stress corrosion
susceptibility is markedly lower in marine atmospheres. The relative resistance to stress corrosion cracking of the brasses is as follows:
Low resistance:
•
Brasses containing more than 15% zinc
•
Brasses containing more than 15% zinc and small amounts of
•
lead, tin, or aluminum
Intermediate resistance:
•
Brasses containing less than 15% zinc
•
The admiralty and the naval brasses are resistant to de-alloying as a
result of the addition of tin. High-zinc brasses resist sulfides better than
low-zinc brasses.