Forms of Metallic Corrosion
57
A plug of dezincified brass may fall out, leaving a hole; whereas a water
pipe having layer-type dezincification may split open. Conditions that favor
selective leaching are:
1. High temperatures
2. Stagnant solutions, especially if acidic
3. Porous inorganic scale formation
Brasses that contain 15% or less zinc are usually immune. Dezincification can be
suppressed by alloying additions of tin, aluminum, arsenic, or phosphorus.
Selective leaching of aluminum takes place in aluminum bronze exposed
to hydrofluoric acid or acid containing chlorides. Copper-aluminum alloys
containing more than 80% aluminum are particularly susceptible. Severe
leaching of tin in tin bronzes in hot brine or steam, and of silicon from silicon
bronzes in high-temperature steam are other examples.
Selective leaching of iron in gray cast iron is observed in gray cast iron
pipes buried in soil or manhole covers exposed to mildly corrosive waters.
Iron leaches out selectively, leaving behind graphite flakes as a porous mass
on the metal surface. The entire structure is weakened although there is little change in metal thickness. Being weakened, fluid pressure from within
or impact from outside will result in failure. Graphite corrosion does not
occur in ductile iron or malleable iron. This form of corrosion is sometimes
referred to as “graphitization.”
3.9.1 Preventive Measures
Preventive measures that may be taken include:
1. Use a more resistant alloy. This is the more practical approach. Red
brass with less than 15% zinc is almost immune. Cupro nickels provide a better substitute in severely corrosive conditions.
2. Periodically remove scales and deposits from the inside surfaces of
pipelines.
3. Remove the stagnation of corrosives, particularly acidic ones.
4. Use cathodic protection.
3.10 Hydrogen Damage
The degradation of the physical and mechanical properties resulting from
the action of hydrogen is known as hydrogen damage. The hydrogen may be
57
A plug of dezincified brass may fall out, leaving a hole; whereas a water
pipe having layer-type dezincification may split open. Conditions that favor
selective leaching are:
1. High temperatures
2. Stagnant solutions, especially if acidic
3. Porous inorganic scale formation
Brasses that contain 15% or less zinc are usually immune. Dezincification can be
suppressed by alloying additions of tin, aluminum, arsenic, or phosphorus.
Selective leaching of aluminum takes place in aluminum bronze exposed
to hydrofluoric acid or acid containing chlorides. Copper-aluminum alloys
containing more than 80% aluminum are particularly susceptible. Severe
leaching of tin in tin bronzes in hot brine or steam, and of silicon from silicon
bronzes in high-temperature steam are other examples.
Selective leaching of iron in gray cast iron is observed in gray cast iron
pipes buried in soil or manhole covers exposed to mildly corrosive waters.
Iron leaches out selectively, leaving behind graphite flakes as a porous mass
on the metal surface. The entire structure is weakened although there is little change in metal thickness. Being weakened, fluid pressure from within
or impact from outside will result in failure. Graphite corrosion does not
occur in ductile iron or malleable iron. This form of corrosion is sometimes
referred to as “graphitization.”
3.9.1 Preventive Measures
Preventive measures that may be taken include:
1. Use a more resistant alloy. This is the more practical approach. Red
brass with less than 15% zinc is almost immune. Cupro nickels provide a better substitute in severely corrosive conditions.
2. Periodically remove scales and deposits from the inside surfaces of
pipelines.
3. Remove the stagnation of corrosives, particularly acidic ones.
4. Use cathodic protection.
3.10 Hydrogen Damage
The degradation of the physical and mechanical properties resulting from
the action of hydrogen is known as hydrogen damage. The hydrogen may be