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

THANK YOU