CORROSIONFACTORS INFLUENCING THE RATE OF
CORROSION
CORROSION
Corrosion is the deterioration of materials by chemical interaction with their environment.
Recipe for corrosion
1 Active metal 2 Water
3 Oxygen (atmospheric corrosion)
4 Acid (chemical corrosion)
5 Salt
6 High temperature
The_Basics_of_How_Corrosion_Occurs.mp4
EFFECTS OF CORROSION1 COLLAPSE OF SHIPS
2 SHUT DOWN OF FACTORIES
3 LOSS OF EFFECIENCY
4 REDUCED VALUES OF GOOD
5 LOST OF SUR FACE PROPERTIES
6 REDUCED STRENGTH
7 ESCAPE OF FLUIDS
TYPES OF CORROSION 1 CREVICE CORROSION
PITTING CORROSION
GALVONIC CORROSION
1 Required ConditionsTwo dissimilar metals in contactOxygen dissolved in water
2 Both Cu and Zn are oxidized in the presence of oxygen in water.
3 The electric potential of copper is 1.90 and for zinc is 1.65.
4 Electrons flow from least potential to higher potential.
Copper does not corrode and zinc corrodes at an accelerated pace.
CORROSION PROTECTIONPreventing the Corrosion of Iron (cathodic protection/sacrificial
anode
COATINGS
Impressed Current Cathodic Protection
Power Supply
Shipboard Power
Controller
Shipboard Power
Insulation
Pt Anode
Reference Electrode
PaintLayer
Hull
e-e-
CORROSION RATE LOSS OF MATERIAL PER
UNIT TIME IS CALLED CORROSION RATE.
mdd = weight loss in mg surface area in dm2 immersion period
INFLUENCING THE RATE OF CORROSION NATURE OF METALS RELATIVE AREAS: WHEN THE
AREA OF CATHODE IS
LARGER AND THE AREA OF
ANODE IS SMALLER
OVER VOLTAGE: CORROSION
RATE IS INVERSELY PROPORTIONAL TO THE OVER VOLTAGE OF A METAL IN THE CORROSIVE ENVIRONMENT
CR= 1 OVER VOLTAGE
NATURE OF METALPOSITION IN EMF
SERIES: CORROSION
DEPENDS ON THE POSITION OF THE MATEL IN THE EMF SERIES.HYDROGEN IN EMF SERIES GETS GREATER CORRODING NATURE VIGOROUSLY.
PURITY OF METAL: THE METAL
SHOULD BE PURE IN STATE . IMPURITIES PRESENT IN THE METAL
ENCOURAGE GALVANIC CORROSION .
NATURE OF THE SURFACE FILM THE NATURE OF THE FILM FORMED ON THE METAL SURFACE WHICH CAN BE DECIDED BY
PILLING - BEDWORTH RULE. 1 ALKALI AND ALKALINE EARTH METALS SUCH AS Mg,CA
etc..FORM OXIDE ,WHOSE VOLUME IS LESS THAN THE VOLUME OF THE METAL
2 HEAVY METALS LIKE Al , Cr ,etc. FORM OXIDE ,WHOSE
VOLUME IS GREATER THAN THAT OF THE MATEL
NATURE OF THE CORROSION PRODUCT IF THE CORROSION
PRODUCT IS SOLUBLE IN THE CORRODING MEDIUM ,THE
CORROSION RATE WILL BE FASTER . SIMILARLY ,IF THE CORROSION PRODUCT IS
VOLATILE , THE CORROSION RATE WILL BE INCREASE.
NATURE OF THE ENVIRONMENTa) TEMPERATURE : AS TEMPERATURE INCREASES,RATE OF CORROSION INCREASES
b) HUMIDITY: AS IN THE ENVIRONMENT INCREASES
CORROSION RATE INCREASES CORROSION INCREASES
NATURE OF THE ENVIRONMENTc) PRESENCE OF CORROSIVE GASES: THE ACIDIC GASES LIKE , (CO)2,(SO)2,H2S AND FUMES OF
HCL,H2SO4,etc..PRODUCE ELECTROLYTES ,WHICH ARE ACIDIC .SO THE ELECTROCHEMICAL CORROSION INCREASES .
d)EFFECT OF PH Formation of hydrogen at lower pH means
that depolarization with oxygen and hydrogen evolution are both playing a role.
POURBAIX DIAGRAM1 EXPLAINS THE CONDITIONS
WHICH A METAL UNDERGOES CORROSION
2 IT IS THE EXAMPLE OF IRON SYSTEM
3 IN Eg : a) INCREASING THE
ELECTODE POTENTIAL b)DECREASING ELECTRODE
POTENTIAL c)INCREASING PH VALUE4 DEPENDING ON
EXPERIMENTAL CONDITIONS ,IRON,WILL BE PASSIVE OR UNDER GO CORROSION
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