Atmospheric Corrosion
113
To be classified as copper, the compound must contain a minimum of 99.9%
copper. Elements such as silver, arsenic, antimony, phosphorus, tellurium,
nickel, cadmium, lead, sulfur, zirconium, magnesium, boron, and bismuth
may be present singularly or in combination.
Copper is noble to hydrogen in the electromotive force (emf) series and
thermodynamically stable with no tendency to erode in water and in nonoxidizing acids free of oxygen. With copper and its alloys, the predominant
cathode reaction is the reduction of oxygen to form hydroxide ions. Therefore,
the presence of oxygen or other oxidizing agents is necessary for corrosion
to take place.
The coating of corrosion products forming the so-called patina has a complex composition that varies from one location to another. Primary components are generally copper oxide and one or more basic copper salts:
Basic sulfate: Cu (OH) 1:5 (SO 4 ) 0.25 , Cu(OH) 1.33 (SO 4 ) 0.33
Basic chloride: Cu(OH) 1.5 Cl 0.5
Basic carbonate: CuOH(CO 3 ) 0.5 , Cu(OH) 1.33 (CO 3 ) 0.33 , CuOH 0.67 (CO 3 ) 0.6
Basic nitrate: Cu(OH) 1.5 (NO 3 ) 0.5
Basic sulfate is predominant in urban atmospheres and basic chloride in
marine atmospheres, whereas in rural atmospheres the primary ingredient
is usually basic sulfate. At times, basic carbonate is found. The presence of
basic copper nitrate indicates that the water film on the metal surface contains appreciable amounts of nitrate.
In most cases, the first products formed are Cu 2 O and CuO. These copper
oxides react slowly with pollutants in the air such as SO x , Cl − , CO 2 , and NO 2
forming the basic salts. For this reaction to proceed, the pH value of the surface moisture must be sufficiently high:
Cu O+0.5SO +1.5H O+0.75O
2Cu(OH) (SO )
2
2
2
2
1.5
4 0.
→
2 25
2
2
2
1 .5
0.5
Cu O+Cl +1.5H O+0.25O
2Cu(OH) Cl
−
→
Corrosion rates for copper in various atmospheres have been determined
as follows:
Atmosphere
Corrosion
Rate (µm/yr)
Rural
1
Urban and industrial
1–3
Marine
1–2
113
To be classified as copper, the compound must contain a minimum of 99.9%
copper. Elements such as silver, arsenic, antimony, phosphorus, tellurium,
nickel, cadmium, lead, sulfur, zirconium, magnesium, boron, and bismuth
may be present singularly or in combination.
Copper is noble to hydrogen in the electromotive force (emf) series and
thermodynamically stable with no tendency to erode in water and in nonoxidizing acids free of oxygen. With copper and its alloys, the predominant
cathode reaction is the reduction of oxygen to form hydroxide ions. Therefore,
the presence of oxygen or other oxidizing agents is necessary for corrosion
to take place.
The coating of corrosion products forming the so-called patina has a complex composition that varies from one location to another. Primary components are generally copper oxide and one or more basic copper salts:
Basic sulfate: Cu (OH) 1:5 (SO 4 ) 0.25 , Cu(OH) 1.33 (SO 4 ) 0.33
Basic chloride: Cu(OH) 1.5 Cl 0.5
Basic carbonate: CuOH(CO 3 ) 0.5 , Cu(OH) 1.33 (CO 3 ) 0.33 , CuOH 0.67 (CO 3 ) 0.6
Basic nitrate: Cu(OH) 1.5 (NO 3 ) 0.5
Basic sulfate is predominant in urban atmospheres and basic chloride in
marine atmospheres, whereas in rural atmospheres the primary ingredient
is usually basic sulfate. At times, basic carbonate is found. The presence of
basic copper nitrate indicates that the water film on the metal surface contains appreciable amounts of nitrate.
In most cases, the first products formed are Cu 2 O and CuO. These copper
oxides react slowly with pollutants in the air such as SO x , Cl − , CO 2 , and NO 2
forming the basic salts. For this reaction to proceed, the pH value of the surface moisture must be sufficiently high:
Cu O+0.5SO +1.5H O+0.75O
2Cu(OH) (SO )
2
2
2
2
1.5
4 0.
→
2 25
2
2
2
1 .5
0.5
Cu O+Cl +1.5H O+0.25O
2Cu(OH) Cl
−
→
Corrosion rates for copper in various atmospheres have been determined
as follows:
Atmosphere
Corrosion
Rate (µm/yr)
Rural
1
Urban and industrial
1–3
Marine
1–2