POWER ENGINEERING 2A2 EVALUATION EXAM ALL
UPDATED QUESTIONS AND ANSWERS SURE A+
✔✔Explain deposition corrosion. - ✔✔This is another mechanism that produces pitting.
A fluid passing through a pipe may absorb metal ions from the pipe material. When this
fluid flows to a location where the metal is different and more anodic, the absorbed ions
may be deposited on the surface of the new location, which is called plating, and
creates a situation with dissimilar metals. Microscopic galvanic action occurs, causing
pitting of the anodic material.
✔✔Conditions that promote pitting - ✔✔- small, local damage to the protective oxide film
- small, local damage to an applied protective coating
- small irregularities in the metal itself
- free oxygen at the metal surface
- microscopic areas of galvanic action
✔✔What is selective leaching? And what is another name for it? - ✔✔The process
involves one element in an alloy being selectively removed by electrochemical reaction
with the environment. This has the effect of making the alloy weaker, brittle, and porous.
One common example is called dezincification, where the zinc is dissolved out of brass.
Another name for this is dealloying.
, ✔✔Hydrogen Embrittlement - ✔✔The loss or reduction of ductility of a metal alloy (often
steel) as a result of the diffusion of atomic hydrogen into the material. The hydrogen
collects between the metal atoms and distorts their natural structure.
✔✔Hydrogen Blistering - ✔✔Atomic hydrogen diffuses into metallurgical voids present
in steel and forms molecular hydrogen. More hydrogen molecules accumulate to the
point where internal pressure increases and causes blistering to occur.
✔✔Hydrogen Stress Cracking - ✔✔Atomic hydrogen diffuses into the steel and interacts
with the atomic structure of the steel, at or near the grain boundaries of the metal.
Pressure accumulates to the point where the metal structure separates, forming a
crack. If corrosion is also active on the surface, the corrosion cell will migrate into the
crack and widen it, in which the process is called hydrogen stress corrosion cracking.
✔✔Stress Corrosion Cracking (SCC) - ✔✔A delayed failure process in which cracks
start slowly, propagate slowly, but fail suddenly. The tensile stress required for stress
corrosion cracking is usually much less than the yield stress of the metal. The stress
may be produced by an external load, internal pressure, or by residual stresses from the
manufacturing processes.
Three common forms of SCC are chloride SCC, sulphide SCC, and caustic
embrittlement.
✔✔Chloride stress corrosion cracking - ✔✔Stainless steel containing less than 30%
nickel are susceptible. The chloride attacks the metal along the grain boundaries,
causing corrosion in the form of cracks. Three components are needed:
- chloride ions
- oxygen
- the metal must be under tensile stress
The rate at which this occurs will increase if the temperature increases or if the pH of
the environment is less than 7.
✔✔Sulphide Stress Corrosion Cracking - ✔✔This occurs in steel and other high
strength alloys when they are exposed to moist hydrogen sulphide environments.
- hydrogen sulphide must be present
- water must be present
- the metal must be high strength alloy steel under applied or residual tensile stress.
Sulphide ions in the aqueous hydrogen sulphide atmosphere inhibit the recombination
of hydrogen atoms in the metal. The smaller hydrogen atoms then diffuse into the
metal's crystal structure, causing hydrogen induced cracking.
UPDATED QUESTIONS AND ANSWERS SURE A+
✔✔Explain deposition corrosion. - ✔✔This is another mechanism that produces pitting.
A fluid passing through a pipe may absorb metal ions from the pipe material. When this
fluid flows to a location where the metal is different and more anodic, the absorbed ions
may be deposited on the surface of the new location, which is called plating, and
creates a situation with dissimilar metals. Microscopic galvanic action occurs, causing
pitting of the anodic material.
✔✔Conditions that promote pitting - ✔✔- small, local damage to the protective oxide film
- small, local damage to an applied protective coating
- small irregularities in the metal itself
- free oxygen at the metal surface
- microscopic areas of galvanic action
✔✔What is selective leaching? And what is another name for it? - ✔✔The process
involves one element in an alloy being selectively removed by electrochemical reaction
with the environment. This has the effect of making the alloy weaker, brittle, and porous.
One common example is called dezincification, where the zinc is dissolved out of brass.
Another name for this is dealloying.
, ✔✔Hydrogen Embrittlement - ✔✔The loss or reduction of ductility of a metal alloy (often
steel) as a result of the diffusion of atomic hydrogen into the material. The hydrogen
collects between the metal atoms and distorts their natural structure.
✔✔Hydrogen Blistering - ✔✔Atomic hydrogen diffuses into metallurgical voids present
in steel and forms molecular hydrogen. More hydrogen molecules accumulate to the
point where internal pressure increases and causes blistering to occur.
✔✔Hydrogen Stress Cracking - ✔✔Atomic hydrogen diffuses into the steel and interacts
with the atomic structure of the steel, at or near the grain boundaries of the metal.
Pressure accumulates to the point where the metal structure separates, forming a
crack. If corrosion is also active on the surface, the corrosion cell will migrate into the
crack and widen it, in which the process is called hydrogen stress corrosion cracking.
✔✔Stress Corrosion Cracking (SCC) - ✔✔A delayed failure process in which cracks
start slowly, propagate slowly, but fail suddenly. The tensile stress required for stress
corrosion cracking is usually much less than the yield stress of the metal. The stress
may be produced by an external load, internal pressure, or by residual stresses from the
manufacturing processes.
Three common forms of SCC are chloride SCC, sulphide SCC, and caustic
embrittlement.
✔✔Chloride stress corrosion cracking - ✔✔Stainless steel containing less than 30%
nickel are susceptible. The chloride attacks the metal along the grain boundaries,
causing corrosion in the form of cracks. Three components are needed:
- chloride ions
- oxygen
- the metal must be under tensile stress
The rate at which this occurs will increase if the temperature increases or if the pH of
the environment is less than 7.
✔✔Sulphide Stress Corrosion Cracking - ✔✔This occurs in steel and other high
strength alloys when they are exposed to moist hydrogen sulphide environments.
- hydrogen sulphide must be present
- water must be present
- the metal must be high strength alloy steel under applied or residual tensile stress.
Sulphide ions in the aqueous hydrogen sulphide atmosphere inhibit the recombination
of hydrogen atoms in the metal. The smaller hydrogen atoms then diffuse into the
metal's crystal structure, causing hydrogen induced cracking.
