CORROSION Flashcards
when atomic hydrogen (H) penetrates into the material.
HYDROGEN EMBRITTLEMENT
are necessary to
cause hydrogen embrittlement
Tensile stresses, susceptible material, and the presence of hydrogen
How to prevent hydrogen embrittlement?
Control of stress level (residual or load) and hardness. • Avoid the hydrogen source. • Baking to remove hydrogen.
best measure to take to reduce or completely eliminate stress corrosion
lower the magnitude of the stress
termed stress corrosion cracking
STRESS CORROSION
results from the combined
action of an applied tensile stress and a corrosive environment; both
STRESS CORROSION
stainless steels stress corrode in solutions containing
chloride ions,
brasses are especially vulnerable when exposed to
ammonia.
arises from the combined action of chemical attack and mechanical
abrasion or wear as a consequence of fluid motion.
EROSION CORROSION
commonly found in piping,
EROSION CORROSION
Propellers, turbine blades, valves, and pumps are also susceptible to this form of corrosion
EROSION CORROSION
One of the best ways to reduce erosion corrosion
change the design to eliminate
fluid turbulence and impingement effects.
found in solid solution alloys and occurs when one element or
constituent is preferentially removed as a consequence of corrosion processes.
SELECTIVE LEACHING
Dealloying, selective leaching and graphitic corrosion can be prevented through the
following methods
Select metals/alloys that are more resistant to dealloying
• Control the environment to minimize the selective leaching
Use sacrificial anode cathodic protection or impressed current cathodic protection
occurs preferentially along grain
boundaries for some alloys and in specific environments.
INTERGRANULAR CORROSION
This type of corrosion is especially prevalent in some stainless steels.
INTERGRANULAR CORROSION
Stainless steels may be protected from intergranular corrosion by the following
measures:
sensitized material to a high-temperature heat treatment in which all the chromium carbide particles are re dissolved,
another form of much localized corrosion attack in which small pits or holes
form.
PITTING CORROSION
It is an extremely insidious type of corrosion, often going undetected and with very little material loss until failure occurs.
PITTING CORROSION
Pitting corrosion can be prevented through:
Proper selection of materials with known resistance to the service environment • Control pH, chloride concentration and temperature • Cathodic protection and/or Anodic Protection • Use higher alloys (ASTM G48) for increased resistance to pitting corrosion
to prevent crevice corrosion
Use welded butt joints instead of riveted or bolted joints in new equipment • Eliminate crevices in existing lap joints by continuous welding or soldering • Avoid creating stagnant conditions and ensure complete drainage in vessels • Use solid, non-absorbent gaskets such as Teflon. • Use higher alloys (ASTM G48) for increased resistance to crevice corrosion
The major factors influencing crevice corrosion are:
crevice type: metal-to-metal, metal-to-non-metal • crevice geometry: gap size, depth, surface roughness • material: alloy composition (e.g. Cr, Mo), structure • environment: pH, temperature, halide ions, oxygen
consequence of concentration differences
of ions or dissolved gases in the electrolyte solution and between two regions of the same metal piece.
Electrochemical corrosion
solution becomes stagnant and there is localized depletion of dissolved oxygen.
Crevice corrosion
prevent galvanic corrosion
• Avoid threaded joints for materials far apart in the galvanic series.
alloys near the top of galvanic series
Cathodic and unreactive
alloys bottom of galvanic series
Anodic
occurs when two metals or alloys having different compositions are
electrically coupled while exposed to an electrolyte.
Galvanic corrosion occurs
experiences corrosion in galvanic
less noble or more reactive metal
Inert metal protected from from corossion
Inert metal, cathode
prevent uniform corrosion?
Use thicker materials for corrosion allowance • Use paints or metallic coatings such as plating, galvanizing or anodizing • Use Corrosion inhibitors or modifying the environment • Cathodic protection (Sacrificial Anode or Impressed Current -ICCP) and Anodic Protection
is a form of electrochemical corrosion that occurs with equivalent intensity
over the entire exposed surface and often leaves behind a scale or deposit.
UNIFORM ATTACK
reactions occur randomly over the surface.
UNIFORM ATTACK
most common form of corrosion
UNIFORM ATTACK
is defined as the destructive and unintentional attack on a metal;
Corrosion
Metallic corrosion is sometimes classified into eight forms:
uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion-corrosion, and stress corrosion.
