Chapter 8: Corrosion and Degradation of Materials Flashcards
there is actual
material loss either by
dissolution (corrosion) or by
the formation of nonmetallic
scale or film (oxidation).
metals
are relatively resistant to deterioration, which
usually occurs at elevated temperatures or in rather extreme
environments
Ceramic materials
For polymers, mechanisms and consequences differ from those for
metals and ceramics, and the term ___ is most frequently
used
degradation
Polymers may dissolve when exposed to a liquid solvent, or they may
absorb the solvent and swell; also, ___
(primarily ultraviolet) and heat may cause alterations in their
molecular structures
electromagnetic radiation
*___ of an object occurs as a result of the interaction between the environment or with the materials that form the object
*several ways in which ceramics break down physically and chemically
degradation
it is in the nature of all the materials used for construction to eventually ___ and ___
degrade and deteriorate
the destructive electrochemical attack of a material
corrosion
why metals corrodes?
redox reaction (reduction-oxidation)
two reactions necessary in corrosion of zinc in acid
oxidation reaction and reduction reaction
occurs when the iron in the metal oxidises
steel rusting
solution for steel rusting
alloying with chromium (stainless steel)
painting/coating the metal
generated by
coupling to the standard
hydrogen electrode, standard
half-cells for various metals
and ranking them according
to measured voltage
electromotive force (emf)
the rate of
material removal as a consequence of
the chemical action, is an important
corrosion parameter
corrosion penetration rate (CPR)
The variables in the corrosion environment, which include ___ can have a decided
influence on the corrosion properties of the materials that are in
contact with it.
fluid velocity, temperature, and composition
In most instances, increasing fluid velocity enhances the rate of
corrosion due to
erosive effects
Increasing the concentration of the corrosive species (e.g., H+
ions in acids) in many situations produces a more rapid rate of
corrosion
forms of corrosion
stress corrosion
erosion-corrosion
pitting
crevice
galvanic
intergranular
selective leaching
uniform attack
work together at crack tips
stress corrosion
break down of passivating layer by erosion (pipe elbows)
erosion-corrosion
downward propagation of small pits & holes
pitting
between two pieces of the same metal
crevice
Dissimilar metals are physically joined. The more anodic one corrodes
galvanic
corrosion along grain boundaries, often where special phases exist
intergranular
preferred corrosion of one element/constituent
selective leaching
oxidation & reduction occur uniformly over surface
also known as general attack corrosion
most common type of corrosion and is caused by a chemical or electrochemical reaction
greater amount of corrosion destruction but safe form of corrosion
predictable, manageable and preventable
uniform attack
it specifically attacks one area of the metal structure
localized corrosion
classification of localized corrosion
pitting corrosion
crevice corrosion
filiform corrosion
results when a small hole, or cavity, forms in the metal, usually as a result of de-passivation of a small area
produces localized galvanic reaction
often difficult to detect due to the fact that it is relatively small
pitting corrosion
similar to pitting
it occurs at a specific location
often associated with stagnant micro-environment
crevice corrosion
occuring under painted or plated surfaces when water breaches the coating
begins with small defects in the coating and spreads to cause structural weakness
filiform corrosion
occurs when two different metals are located together in a corrosive electrolyte
forms between two metals, where one becomes anode and one becomes cathode
galvanic corrosion
three conditions must exists for galvanic corrosion to occur
-electrochemically dissimilar metals must be present
-the metals must be in electrical contact
-the metals must be exposed to an electrolyte
is a corrosion process that can result from a combination of environmental conditions affecting the metal
environmental cracking
types of environmental cracking
-stress corrosion cracking (SCC)
-corrosion fatigue
-hydrogen-induced cracking
-liquid metal embrittlement
___ or flow-accelerated corrosion, results when a protective layer of oxide on a metal surface is dissolved or removed by wind or water, exposing the underlying metal to further corrode and deteriorate
flow-assisted corrosion (FAC)
-occurs as a result of repeated wearing, weight and/or vibration on an uneven, rough surface
-often found in rotation and impact machinery
fretting corrosion
-is a chemical or electrochemical atttack on the grain boundaries of a metal
-often occurs due to impurities in the metal which tend to be present in higher contents near grain boundaries and these boundaries can be more vulnerable to corrosion than the bulk of the metal
transgranular and intergranular corrosion
de-alloying also called
selective leaching
can also be caused by high temperature oxidization, sulfidation and carbonization
high temperature corrosion
-is the selective corrosion of a specific element in an alloy
-the most common type of it is de-zincification of unstabilized brass
-the result of corrosion in such cases is a deteriorated and porous copper
de-alloying
corrosion prevention
-through the judicious selection of materials
-cost may be a significant factor
-changing the character of the environment
-lowering the fluid temperature and/or velocity
-increasing or decreasing the concentration of some species
are substances or mixtures that in low concentration and in aggressive environment inhibit, prevent or minimize the corrosion
inhibitors
3 mechanism of the inhibitor
-inhibitor is chemically adsorbed on the surface of the metal
-inhibitor leads a formation of a film by oxide protection of the base metal
-inhibitor reacts with a potential corrosive component
examples of application of inhibitors
petroleum industry
packaging industry
sour gas systems
potable water systems
engine coolants
environmental types of corrosion inhibitor
neutralizer
scavanger
adsorption (mixed) 2 types of corrosion inhibitor
chemical
physical
inorganic 2 types
anodic
cathodic
green 2 types of corrosion inhibitor
extract
oil
2 cathodic types of corrosion inhibitor
poisonous
precipitation
