C O R R O S I O N P R E V ENT I O N T E CHNI Q UE S Flashcards
The most common method of preventing
corrosion is the
selection of proper metal
or alloy
Not stainless; Not the most
corrosion-resistant material
stainless steel
t is a generic name for a
series of more than 30
different alloys containg 11.5
to 30% chromium and 0 to
22% nickel, together with
other alloy addition.
stainless steel
Stainless steel have widespread applications in resisting
corrosion but it should be remembered that
____________
they do not resist all corrosives.
stainless steel are
less resistant than ordinary structural steel:
Chloride-containing medium
Stressed structures
also more susceptible
to localized corrosion than ordinary
structural steels
Stainless steel
Frequently, the quality of stainless steel is checked
with a magnet
__________ stainless steels represent good alloys
and stainless steels that are magnetic are inferior
nonmagnetic
There is ___________ between magnetic
susceptibility and corrosion resistance
no correlation
A large number of corrosion failures
can be directly attributed in the
Indiscriminate selection of stainless steels
The corrosion resistance of a_________is
usually better than that of one containing
impurities or small amounts of other
elements
pure metal
are usually
expensive and relatively soft and weak
pure metals
Not expensive in a fairly pure
state - 99.5% plus.
Used for handling hydrogen
peroxide
Aluminum
More corrosion-resistant
than induction-melted
zirconium.
Used in atomic-energy
application.
ARC-MELTED ZIRCONIUM
This category involves integral or solid
nonmetallic construction and also sheet
linings or coverings of substantial
thickness
Nonmetallics
The five general classes of nonmetallics
are
rubbers, natural, and synthetic;
plastics;
ceramics;
carbon and graphite;
and wood.
Compared with metals and
alloys: weaker, softer, and
more resistant to chloride
ions but less resistant to
strong sulfuric acid and
oxidizing acids.
rubbers and plastics
Has excellent corrosion and
high-temperature resistance.
Brittle and has lower tensile
strength.
ceramics
Good corrosion resistance,
electric and heat conductivity
Fragile
carbon
frequently as
important as the choice of materials of
construction.
design of a structure
Design should consider ________ together with an _____ for corrosion
mechanical and strength requirement, allowance
Corrosion is a penetrating action. Hence, it is
necessary to make allowances for the reduction of _________ n designing pipes, tanks, and other
components that can corrode
thickness
is usually made twice the
thickness that would give the desired life.
Wall thickness
Wall thickness must meet the mechanical
requirements such as
pressure, weight, and stress
considerations
________rather than
rivet tanks and
other containers
Wield
Design tanks and
other containers for
easy draining, easy cleaning
Provide for __________
with dry air or inert gas if
vessels “inhale” moist marine
atmosphere while being
emptied.
blanketing
heterogeneity must be _____
avoided
Design to ______ air
exclude
As a result of laboratory experiments in
salt water, __________ that copper could be successfully
protected against corrosion by coupling it
to iron or zinc
Sir Humphry Davy 1824
is a technique used to control
the corrosion of a metal surface by making it the cathode of an electrochemical cell
Cathodic protection
achieved by
supplying electrons to the metal structure
to be protected
Cathodic protection
f current is considered to flow from ( + ) to ( -‘ ), as
in conventional electrical theory, then a structure is
protected _______________
if current enters it from the electrolyte
Types of cathodic protection
Galvanic/Sacrificial
Impressed current method
This type of CP uses a more reactive metal (the sacrificial
anode) that corrodes preferentially to the metal structure (the
cathode) that needs to be protected
Galvanic/Sacrificial
_______protecting the underlying metal will continue to
degrade over time until the sacrificial anode is no longer capable of
supplying protection.
sacrificial anode
Sacrificial metals used for cathodic protection consist of
magnesium-base and aluminum-base alloys and, to a lesser
extent, zinc
serve essentially as sources of
portable electrical energy
sACRIFICIAL ANODES
The open - circuit potential difference of magnesium with
respect to steel is about
1V (1.3 if in seawater)
The potential of zinc is less
than that of magnesium
-0.8V in seawater
as a
sacrifi cial anode is that it tends to
become passive in water or in soils
with accompanying shift of
potential to a value approaching
that of steel
aluminum
Advantages of galvanic/sacrificial method
No external power source is required so it can be used remotely.
It has a lower installation cost.
Minimum maintenance is required.
Systems seldom cause adverse effects
(interference) on their structure
Disadvantages of galvanic/sacrificial method
There is limited driving potential and current output.
It can be ineffective in high resistivity environments.
It is not generally applicable for poorly coated
structures.
The life of the anodes tends to be relatively short,
depending on load.
Its existence is easily forgotten by maintenance
personne
This type of CP uses an external power
source (impressed current) to provide the
electrons required to cathodically protect
the structure.
IMPRESSED CURRENT METHOD
The negative terminal
of the power supply is connected to the __________,
and the positive to an inert anode such as
________
tank, graphite or Duriron
The anode usually surrounded by ____________, which improves electric contact
between the anode and the surrounding soil.
backfill
consisting of coke breeze, gypsum, or
bentonite
Aggressive corrosives such as hot acids
require prohibitively___________currents, whereas
much lower currents are needed to protect
steel in less severe environments
(concrete)
high
FACTORS THAT INFLUENCE CURRENT
REQUIREMENTS
Environment
Coating
Pipes with ________ require much lower
currents since the only areas requiring protection
are defects or “holidays” in the protective layer
organic coating
Of the sacrificial anodes, _________ is
the most widely used.
magnesium
are the
most widely used anode materials for impressed anodes.
Steel, graphite, and silicon-iron
Practical application of anodic protection
and use of the potentiostat was first
suggested by
Edeleanu.
n is the formation
of a protective film on metals by
externally applied anodic currents
Anodic protection
achieved by exhibiting
an active-passive behavior in order to
form a passive film
Anodic protection
Anodic protection is an ____ for maintaining
passive a metal with an active–passive behavior
when placed in an aggressive environment
electrochemical technique
It is an electronic device that maintains a metal at a
constant potential with respect to a reference electrode
in order to anodically protect a structure
potentiostat
The terminals of a potentiostat is
connected to the tank,
auxiliary cathode,
reference electrode.
Advantages of anodic protection
Applicability in extremely corrosive
environments.
Low current requirements
Limitation of anodic protection
Only applicable to passive metals and
alloys
known for its
excellent corrosion resistance
properties, particularly against
corrosive environments like sulfuric acid.
Type 316 SS
Type 316 SS contains _______, which
enhances its resistance to pitting and
crevice corrosion, making it suitable for
such harsh environments.
molybdenum
