Chapter 16. Corrosion and Degradationof Materialsof Materials

16.1 Introduction

With a knowledge of the types of and an understanding of the mechanisms andWith a knowledge of the types of and an understanding of the mechanisms andcauses of corrosion and degradation, it is possible to take measures to prevent them from occurring.

Deteriorative mechanisms are different for the three materials. In metals, thereis actual material loss either by dissolution (corrosion) or by the formation of nonmetallic scale or film (oxidation)( )

For Polymer, degradation is used. Swelling in solvent, alterations in molecular Structure due to electromagnetic radiation and heat.

16.2 Electrochemical considerations

Oxide reaction : anode

−+ +→ neMM n

−+ +→ eFeFe 22

reduction reaction : cathode

222 HeH →+ −+

OHeHO 244 →++ −+)(2 2

2 gasHZnHZn +→+++

)(442 22−− →++ OHeOHO

OHeHO 22 244 →++

MneM n →+ −+

Electrode Potentials

fThe magnitude of the electrical potential that is established between anode and cathode is indicative of the driving force for the corrosion reaction

++ +→+ 22 FeCuFeCu ++ +→+ 22 ZnFeZnFe

0.780V 0.323V

Galvanic Corrosion Cells

Corrosion couples, called galvanic cells :(1) Composition cells (2) stress cells, and (3) concentration cells

- Composition cells : any two dissimilar metals

Two phase alloy is more subject to corrosion than is a single-phase alloy.

Al-Si alloy

Heat treatment may affect the corrosion rate by altering the microstructureof metal.

- Stress cells : do not compositional difference, they involve dislocations,grain boundaries and highly stress regions g g y g(high energy state serve as anode )

- Concentration cells : difference in electrolyte compositions.(low concentration serve as anode )

Local Corrosion

- Corrosion seldom progress uniformly into a metal surface.Rather, it acts locally.

- Local area has less oxygen serves as anode. The more exposed areasyg pbecome the cathode.

Galvanic Protection

We can achieve service protection against corrosion in some applicationsby arranging for the product to be the cathode

Degradation of Polymers

S lli d di l tiSwelling and dissolution

Bond rupture

Radiation effects, chemical reaction effects, thermal effects, ,