lec corrosion 13
TRANSCRIPT
-
7/30/2019 Lec Corrosion 13
1/70
CORROSION OF METALS
Introduction
Definition
Economic impact
Types of Corrosion
Combating corrosion
Anti corrosive coatingsand Paints
http://en.wikipedia.org/wiki/Image:Rust_and_dirt.jpg -
7/30/2019 Lec Corrosion 13
2/70
The diffuse dispersion of metals in the society
has many sources.
-
7/30/2019 Lec Corrosion 13
3/70
Corrosion - IntroductionAny material which serves the humans needs such as
housing, food, clothing, communication, transportation,energy, socioeconomic development has the inherent
tendency to decay.
Virtually no engineering material is stable. The rate ofdecay varies depending upon the material and its
environment.
The basic cause of corrosion is the instability of metals intheir refined forms.
The metals tend to revert to their natural states through
the process of corrosion..
-
7/30/2019 Lec Corrosion 13
4/70
No material is inert!
All metals/alloys interact with the environment.
SO2
Metal runoff/release
Me, Men+
-
7/30/2019 Lec Corrosion 13
5/70
Corrosion process
-
7/30/2019 Lec Corrosion 13
6/70
ContdCorrosion - IntroductionHow to stop this reversion?
In order to stop reversion to the natural state, engineeringmaterials require
protective coatings,
inhibitors,
alloy additions,
design procedures,
maintenance and refurnishing.
It is good to extend the life of materials, keep them corrosion
free as possible.
Extending the life of a 1000 megawatt nuclear power plant for
one day would mean a saving of 0.5 mill dollars.
-
7/30/2019 Lec Corrosion 13
7/70
What is Corrosion ?
Corrosion is the gradual physico -
chemical destruction of materials by the
attacking action of the environments.
The word corrosion originated from
the Latin word corrodereto gnaw
away.
-
7/30/2019 Lec Corrosion 13
8/70
Corrosion may be defined as the
destruction of material by chemical,
electrochemical, or metallurgical
interaction between the environment andthe material.
The term corrosion is sometimes also applied to
the degradation of plastics, concrete and wood,
but generally refers to metals.
Definition of Corrosion
-
7/30/2019 Lec Corrosion 13
9/70
Impacts of corrosion
Losses are economic and safety:
Reduced Strength
Downtime of equipment Escape of fluids
Lost surface properties
Reduced value of goods
-
7/30/2019 Lec Corrosion 13
10/70
Impacts of corrosion
The consequences of corrosion are many and varied
and
the effects of these on the
safe,
reliable and
efficient operation of equipment or structures
are often more serious than the simple loss of a mass
of metal.
Failures of various kinds and the need for expensive
replacements may occur even though the amount of
metal destroyed is quite small.
-
7/30/2019 Lec Corrosion 13
11/70
Economic Impact
Corrosion a most serious problems ofindustry
Corrosion causes damage in the billions of
dollars each year. Apporx. 10% world metal
production is completely lost due to
corrosion each year.
According to estimate national annual costs
of corrosion to a country vary from 1 % to 4 %
of GNP.
-
7/30/2019 Lec Corrosion 13
12/70
Economic Impact
A study showed that for 1998 the totalannual direct cost of corrosion in the US
was $276 billion ( 3.1% of the US
GDP.
15% to 25% of this loss can be saved if
available knowledge and techniques are
used.
-
7/30/2019 Lec Corrosion 13
13/70
How much does corrosion really cost?
Before After
Silver Bridge Collapse
December 15, 1967 in Kanauga, OH
Highways and Bridges
-
7/30/2019 Lec Corrosion 13
14/70
Highways & Bridges1: $8.3 Billion per year + safety issues
How much does corrosion really cost
in USA ?
-
7/30/2019 Lec Corrosion 13
15/70
Economic losses
Costs of corrosion are divided into direct & indirect losses
Direct losses
include costs of replacement of corroded structures &
machinery / their components, e.g., condenser tubes,
pipelines, metal roofings, repainting of structures againstrusting.
These losses also include the extra cost of using
- corrosion-resistant material in place of Carbon steel or
other cheaper materials,
- adding inhibitors to enclosed systems,
- protective systems for metal structures.
-
7/30/2019 Lec Corrosion 13
16/70
Economic losses
Indirect losses
Loss of product
Loss of production
Loss of efficiency
Product contamination
Over design
Maintenance of stand-by Machinery & Equipment
General loss e.g., loss of health & life which can
not be computed in terms of money.
-
7/30/2019 Lec Corrosion 13
17/70
Types of Corrosion
Dry corrosion or
oxidation of metals
Wet corrosion or
electrochemical
oxidation
Galvanic Corrosion
Electrode Potential
Examples of
Galvanic Corrosion
Atmospheric corrosion
Pitting Corrosion
Erosion / Cavitation
corrosion
Fretting corrosion
Crevice Corrosion
Stress Corrosion
Intergranular
Corrosion
Microbial Corrosion
-
7/30/2019 Lec Corrosion 13
18/70
Dry Corrosion or Oxidation of Metals(scaling of iron at high temperature)
Many metals tend tooxidize to some extent at
all temp. When iron
heated in the presence of
oxygen it becomes coated
with a film of black scale
as per following reaction:
2Fe + O2 = 2FeO Iron is oxidized as its
atoms lose electrons.
-
7/30/2019 Lec Corrosion 13
19/70
Wet Corrosion or Electrochemical Oxidation
Iron does not rust
in a completely dry atmosphere,
nor will it rust
in completely pure, O2-free water,
but in a moist atmosphere, as a result,
reddish-brown deposit of ferric hydroxide soon begins to
develop, as :
4Fe + 6H2O + 3O2 = 4Fe(OH)3
-
7/30/2019 Lec Corrosion 13
20/70
Contd Wet Corrosion or
Electrochemical Oxidation
The fundamental principle is that atoms
of iron in contact with oxygen and water
oxidize, i.e., they lose electrons and
enter into solution as ferrous ions (Fe++)
which are further oxidized to ferric ions
(Fe+++).
-
7/30/2019 Lec Corrosion 13
21/70
Ranks the reactivity of metals/alloys in seawater
PlatinumGold
Graphite
Titanium
Silver
316 Stainless Steel
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless SteelIron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
8
Galvanic Series
-
7/30/2019 Lec Corrosion 13
22/70
Electrode Potential for some metals
-
7/30/2019 Lec Corrosion 13
23/70
Galvanic Corrosion
Galvanic Corrosion occurs when two
dissimilar metals electrically contacteach other and are immersed in an
electrolyte.
In order for galvanic corrosion to
occur, an electrically conductive path
and an ionically conductive path are
necessary.
This affects a galvanic couple wherethe more active metal corrodes at an
accelerated rate and the more noble
metal corrodes at a retarded rate
-
7/30/2019 Lec Corrosion 13
24/70
Requirements for Corrosion:
In order for galvanic corrosion to occur, an electrically conductive path and an
ionically conductive path are necessary.
Ionic Current Path
Electronic Path
ANODE CATHODE
Where
Corrosion
Occurs!!!!
-
7/30/2019 Lec Corrosion 13
25/70
Anodic & Cathodic Reactions
-
7/30/2019 Lec Corrosion 13
26/70
Examples of Galvanic corrosion
-
7/30/2019 Lec Corrosion 13
27/70
Galvanic corrosion of pearlite
-
7/30/2019 Lec Corrosion 13
28/70
Galvanic corrosion
an examples of bad plumbing
-
7/30/2019 Lec Corrosion 13
29/70
Atmospheric Corrosion
Most common & economically most costly form. The atmospheric corrosion process depends not
only on the moisture content of air but also on the
gaseous impurities, dust particles and othercontaminants which favour condensation of
moisture on the metal surface.
Atmospheric corrosion rates vary markedly all over
the world depending upon the climatic region,
season of the year, time of the day etc.
-
7/30/2019 Lec Corrosion 13
30/70
Contd Atmospheric Corrosion
Approaching the sea coast, air is laden with increasing
amount ofsea salt.
At industrial areas, appreciable amounts ofSO2which is
converted into H2SO4, and to lesser amounts ofH2S, NH3,
NO2 and various suspended salts are encountered.
A metal resisting one atmosphere may lack effective
resistance elsewhere and hence relative performance of
metals changes with location.
-
7/30/2019 Lec Corrosion 13
31/70
Pitting Corrosion
Pitting is a localized attack, in the
form of pits, such as observed on
stainless steel immersed in
chloride ion solutions.This causes
premature failure.
The main causes of pitting are
galvanic corrosion,
low or high oxygen concentrations
and
applied stresses.
E i i
-
7/30/2019 Lec Corrosion 13
32/70
Erosion corrosion
Erosion is acceleration of
corrosion because of relative
movement between
corrosive fluid and material
as may occur in pumping
equipment.
-
7/30/2019 Lec Corrosion 13
33/70
Cavitation corrosion
Cavitation corrosion is a
form of velocity effect
caused by the explosion of
bubbles formed where the
local pressure in the
flowing fluid drops below
the vapour pressure of the
liquid.
It is observed on ship
propellers, pump impellors
etc.
-
7/30/2019 Lec Corrosion 13
34/70
Fretting
corrosion
Fretting corrosion is a surface damage produced by vibration which
results in striking or rubbing at the interface of close fitting, highly
loaded surfaces.
It is common at surfaces of clamped or press fits, splines, keyways,
and other close-fitting parts subject to minute relative movement. Fretting corrosion ruins bearings, destroys dimensions, and reduces
fatigue strength.
This type of corrosion is a mechanical-chemical phenomenon.
-
7/30/2019 Lec Corrosion 13
35/70
Crevice Corrosion
It is a intense
localized corrosion at
a crevice under a
bolt head or flanged
connections usually
due to concentration
cells.
-
7/30/2019 Lec Corrosion 13
36/70
Crevice Corrosion
St C i
-
7/30/2019 Lec Corrosion 13
37/70
Stress Corrosion
Stress corrosion is caused by
simultaneous presence of tensilestress and a particular corrosive
medium e.g., cracking of cold
worked Cu or brass in ammonia or
stainless steel in chloride-
containing water.
The magnitude of stress
necessary to cause failure dependson the corrosive medium and on
the structure of base metal.
Intergranular Corrosion
-
7/30/2019 Lec Corrosion 13
38/70
Intergranular Corrosion
It occurs in the vicinity of grain boundaries
Due to elevated temperature Chromium Carbides can form in the grain
boundaries of SS.
This Chemical reaction depletes the Cr in the HAZ near the grain
boundary, making those areas much less resistant to corrosion. This
creates a galvanic cell which leads to weld decay.
-
7/30/2019 Lec Corrosion 13
39/70
Intergranular Corrosion
Preferential attack alonggrain boundaries
Results from localized
differences in chemistry
Common in SS, nickel some Alalloys
Sensitive Regions
precipitates
-
7/30/2019 Lec Corrosion 13
40/70
Microbial Corrosion
Microbial or Bacterial Corrosion is
caused or promoted bymicroorganisms.
It can apply to both metals and non-
metallic materials, in both the
presence and lack of Oxygen. Sulfate reducing bacteria are
common in lack of oxygen; they
produce hydrogen sulfide causing
sulfide stress cracking.
In the presence of oxygen, some
bacteria directly oxidize iron to
hydroxide.
Titanics bow exhibiting
microbial corrosion damage in
the form of rusticles
-
7/30/2019 Lec Corrosion 13
41/70
Corrosion of plastics
This dished end of a vessel
is made of glass fibre
reinforced PVC.
Due to internal stresses
and an aggressive
environment it has sufferedenvironmental stress
cracking.
-
7/30/2019 Lec Corrosion 13
42/70
Some examples of corrosion
-
7/30/2019 Lec Corrosion 13
43/70
Galvanic corrosion
This rainwater guttering is made of aluminium andwould normally resist corrosion well. Someone tied
a copper aerial wire around it, and the localised
bimetallic cell led to a knife-cut effect.
-
7/30/2019 Lec Corrosion 13
44/70
Galvanic corrosion
This polished Aluminium
rim was left over
Christmas with road salt
and mud on the rim.Galvanic corrosion has
started between the
chromium plated brass
spoke nipple and the
aluminium rim.
-
7/30/2019 Lec Corrosion 13
45/70
Galvanic corrosion
Galvanic corrosion
can be even worse
underneath the
tyre in bicycles.
Here the corrosion
is so advanced it
has penetrated
the rim thickness.
Erosion corrosion
-
7/30/2019 Lec Corrosion 13
46/70
Erosion corrosion
Erosion-Corrosion
I l C i
-
7/30/2019 Lec Corrosion 13
47/70
Intergranular Corrosion
Occurs in specific alloysprecipitation of corrosive
specimens along grain boundaries and in particular
environments
e.g. : Chromium carbide forming in SS, leaving adjacent areas depleted in Cr
Solutions: High temp heat treat to redissolve carbidesLower carbon content (in SS) to minimize carbide
formation
Alloy with a material that has stronger carbide
formation (e.g., Ti or Nb)
Intergranular Corrosion
-
7/30/2019 Lec Corrosion 13
48/70
Intergranular Corrosion
stainless steel
-
7/30/2019 Lec Corrosion 13
49/70
Influence of corrosion on value
A very slight amount of corrosion may not interfere with the
usefulness of an article, but can affect its commercial value. At thepoints where these scissors were held into their plastic case some
surface corrosion has occurred which would mean that the shop
would have to sell them at a reduced price.
-
7/30/2019 Lec Corrosion 13
50/70
Motor vehicle corrosion and safety
The safety problems associated with corrosion of motor vehicles is
illustrated by the holes around the filler pipe of this petrol tank. The
danger of petrol leakage is obvious. Mud and dirt thrown up from
the road can retain salt and water for prolonged periods, forming a
corrosive poultice.
-
7/30/2019 Lec Corrosion 13
51/70
House Drain andDrain Cap
METAL: Cast Iron
ENVIRONMENT: Residential basement water exposure
FORM OF CORROSION: General
METHOD TO CONTROL! Surface is painted for protection. Note the 1 year old cap shows significant
corrosion already!
1 year old
cap
30 year oldcap
i
-
7/30/2019 Lec Corrosion 13
52/70
Corrosion at sea
Sea water is a highly corrosive electrolyte towards mild steel. Thisship has suffered severe damage in the areas which are most
buffeted by waves, where the protective coating of paint has been
largely removed by mechanical action.
-
7/30/2019 Lec Corrosion 13
53/70
The current trend for
aluminium vehicles is not
without problems. This
aluminium alloy chassis
member shows very
advanced corrosion due to
contact with road salt
from gritting operations or
use in coastal / beach
regions.
Corrosion: an extensive expensive
-
7/30/2019 Lec Corrosion 13
54/70
Corrosion: an extensive, expensive
materials degradation process
Corrosion in a hi h tem erature i eline
A t d hi k
-
7/30/2019 Lec Corrosion 13
55/70
A rusted shipwreck
Forms of Corrosion
-
7/30/2019 Lec Corrosion 13
56/70
Forms of Corrosion
Stainless screw v cadmium plated steel washer
Forms of Corrosion
http://www.corrosion-doctors.org/Aircraft/galvseri-sea.htmhttp://www.corrosion-doctors.org/Aircraft/galvseri-sea.htm -
7/30/2019 Lec Corrosion 13
57/70
Forms of Corrosion
Stress Corrosion Cracking (SCC) in a U-bendsample of 316 stainless steel exposed to
Supercritical Water Oxidation (SCWO)
environment
Exfoliation in aluminum aircraft alloy
-
7/30/2019 Lec Corrosion 13
58/70
Corrosion control / prevention
C b ti C i
-
7/30/2019 Lec Corrosion 13
59/70
Combating Corrosion
Following are the most important methods used
in industry for protection of corrosion:
Use of high purity metals
Use of alloy additions
Use of special heat treatments
Proper design
Cathodic protectionUse of Inhibitors
Surface coatings
-
7/30/2019 Lec Corrosion 13
60/70
Proper Design
-
7/30/2019 Lec Corrosion 13
61/70
Sketch of Cathodic Protection
-
7/30/2019 Lec Corrosion 13
62/70
Sketch of Cathodic Protection
showing pipe, auxiliary anode and rectifier
i i b i hibi
-
7/30/2019 Lec Corrosion 13
63/70
Reactive Coatings by using Inhibitor
If the environment is controlled in re-circulating
systems, corrosion inhibitors (chromates, phosphates,
long change organic molecules) are added to it.
These form an electrically insulating and / or chemically
impermeable coating on exposed metal surfaces, to
suppress electrochemical reaction. Thus corrosion is
reduced or eliminated.
Inhibitors are added to the antifreeze mixtures used in
automobile radiators.
Anti corrosive surface Coatings
-
7/30/2019 Lec Corrosion 13
64/70
Anti-corrosive surface Coatings
Metal surface can be protected by the provision of
following surface coatings:
Paints,
Oxide films and
Metallic coatings
Note: Prior to applying coatings, pretreatments(such as shot/sand blasting, cleaning, pickling,
rinsing, drying etc) of substrate is essential.
Paints
-
7/30/2019 Lec Corrosion 13
65/70
Paints
Paints and other organic coatings are primarily used
to improve to appearance of the surfaces andstructures. Its use for corrosion protection only is
secondary.
Paints [which are mixture of pigments (metallicoxides e.g., TiO2, Pb3O4, Fe2O3 or other compounds
ZnCrO4, PbCO3, BaSO4 etc) suspended in organic or
aqueous vehicle] provide a protective film to themetal and is effective only as long as the film is
unbroken.
-
7/30/2019 Lec Corrosion 13
66/70
Anodizing
Aluminum alloys often undergo a surface treatment. Electrochemical
conditions in the bath are carefully adjusted so the uniform pores
several nanometers wide appear in the metals oxide film. These
pores allow the oxide to grow much thicker then passivating. At theend of the treatment, the pores are allowed to seal, forming a
harder-than-usual surface layer. If the coating is scratched, normal
passivation processes take over to the protect the damaged area
Metallic Coatings
-
7/30/2019 Lec Corrosion 13
67/70
Metallic Coatings
Metallic coating may be obtained by a variety of
methods such as:Diffusion (Carburizing, Cyaniding,
Nitriding, Chromizing)
Hot dipping (mainly coating of Zn, Sn, Pb, Al &Cd on steel)
Galvanizing / Znic coating; used for the
protection of nuts, bolts,
screws, nails etc.
Electroplating
Metallic Coatings
-
7/30/2019 Lec Corrosion 13
68/70
Metallic Coatings
Cladding Coating becomes the integral part of the metal.
In this process, sheets of cladding material are
strapped to an ingot of the base material.
After heating to the rolling temp, the straps are
removed and the entire assembly is rolled.
The heat and pressure during rolling weld the two
materials together, e.g., Alclad or GMCS.
-
7/30/2019 Lec Corrosion 13
69/70
Thanks
-
7/30/2019 Lec Corrosion 13
70/70
Corrosion prevention