c7-corrosion & degradation of materials

8/8/2019 c7-Corrosion & Degradation of Materials

1/21

CORROSION & DEGRADATIONCORROSION & DEGRADATION

OF METALSOF METALS


2/21

CORROSIONCORROSION

Figure 1 is a view of the failure origin showing severe areas ofsurface corrosion. The corrosive environment is the typical

salt/water mixture resulting from the deicing of roads during

the winter.


3/21

DEFINITION OF CORROSION

Slow and progressive destruction of a

metal brought about by the action of

an external agent and various causes

of a mechanical nature.


4/21

Corrosive EnvironmentspH

(Remember! pH 0 to 7: acidic, pH 7 to 14: alkaline)

Lower pH

Higher the amount of free acid present

more corrosive the solution

Aggressive species

Aggressive species are corrosive materials that can penetrate

and brake down protective surface films or modify the shape of

the cathodic polarization curve.

Ex: chloride ion, sulphur ion

Oxidizing species

Oxidizing species exhibit mild to severe effects on corrosiveness.Oxidizing species depolarize corrosion reactions.


5/21

To forms of metal deterioration can

be stated:

DIRECT OXIDATION:

Where the metals atoms combine with theaggressive substance:

2Fe +O2 ------2FeO.


6/21

ELECTROCHEMICAL CORROSIONELECTROCHEMICAL CORROSION

For metals, corrosion process is an electrochemicalprocess where metal will loose electron(s)

M Mn+ + ne-

Fe Fe2+ + 2e- or Al Al3+ + 3e-

The side which oxidation occur

Electron(s) generated from the metal (duringoxidation) must be transferred to form a chemicalspecies.

Process of gaining electron(s)

2H+

+ 2e-

H2 Reduction occurs at

oxidationoxidation

anodeanode

reductionreduction

cathodecathode


7/21

Oxidation:Zn Zn2+ + 2e-

Reduction2H+ + 2e- H2 (gas)

Acid solutionAcid solution


8/21

Standard EMF Series

AuCu

Pb

Sn

Ni

Co

Cd

Fe

Cr

Zn

Al

Mg

Na

K

more cathodic

more anodic

EMF series is gathered bycoupling a standard half cellof a metal to standardhydrogen electrode

EMF series give corrosiontendencies of metals

Those on top >difficult to oxidise

Those on bottom ....>easy to be oxidised


9/21

Uniform AttackOxidation & reduction

occur uniformly over

surface. Selective Leaching

Preferred corrosion ofone element/constituent

(e.g., Zn from brass (Cu-Zn)).

Stress corrosion

Stress & corrosionwork together

at crack tips.

GalvanicDissimilar metals arephysically joined. The

more anodic one

corrodes.

Erosion-corrosionBreak down of passivating

layer by erosion (pipe

elbows).

FORMS OF CORROSION

Formsof

corrosion

Crevice Between twopieces of the same metal.

Fig. 17.15, Callister 7e. (Fig. 17.15 iscourtesy LaQue Center for Corrosion

Technology, Inc.)

Rivet holes

IntergranularCorrosion along

grain boundaries,

often where specialphases exist.

Fig. 17.18, Callister 7e.

attackedzones

g.b.

prec.

Pitting

Downward propagationof small pits & holes.

Fig. 17.17, Callister 7e.(Fig. 17.17 from M.G.Fontana, CorrosionEngineering, 3rd ed.,McGraw-Hill BookCompany, 1986.)


10/21

Erosion-corrosion

Erosion corrosion: acceleration in rate

of corrosion due to relative motionbetween corrosive fluid & surface

Found mostly in piping; especially atbends, elbows or abrupt change in

pipe diameter where fluids becometurbulence or fluid change directionsuddenly


11/21

Pitting corrosion

Localized corrosion attack

Small pits/holes form

Oxidation occurs within the pitsitself

A pit must form from detects likescratches or slight variation incombination

Undetected and suddenunexpected failure


12/21

Crevice corrosion

Corrosion occurs because of different

in concentration electrolyte aroundmetal parts

Occurs under valve, washers,

insulation material, fastener heads,surface deposits, coatings, threads, lap

joints & clamps


13/21

Galvanic corrosion

Two metals or alloys havingdifferent compositions areelectrically coupled while

exposed to an electrolyte

Electrochemical reactionleads to corrosion of metal


14/21

GALVANIC CORROSIONGALVANIC CORROSION

2 dissimilar metal electrodesimmersed in solution of their ownions.

Electrode that has morenegative oxidation potential willbe oxidized.

Zn Zn2+

Cu2+ Cu

oxidized

reduced


15/21

GALVANIC SERIESGALVANIC SERIESranks of the reactivity of metals/alloys in seawater

Galvanic seriesgives the cathodic,

anodic relationshipbetween metalsE.g in seawater, Zinc

is more active thanaluminum

Series is determinedexperimentally forevery corrosiveenvironment

PlatinumGoldGraphiteTitaniumSilver

316 Stainless SteelNickel (passive)CopperNickel (active)Tin

Lead316 Stainless SteelIron / SteelAluminum alloysCadmiumZinc

Magnesium


16/21

Intergranular corrosion

Corrosion occurs along grain

boundaries for alloys in specificconditions

Specimen disintegrates along the

grain boundaries and result in failure


17/21

Corrosion Prevention MaterialsSelections & Design

Choose a right metal once the workingenvironment has been detected. Use less

active metal (galvanic series and EMF) Avoid dissimilar metals that can cause

galvanic corrosion

Provide allowance for corrosion of thickness

Allow complete drainage Make sure parts can be replaced

Weld rather than rivet


18/21

Self-protecting metals!-- Metal ions combine with O

to form a thin, adhering oxide layer that slows corrosion.

C

hange the environment e.g. lowering the fluidtemperature, pressure, concentration and velocity- Reduce T(slows kinetics of oxidation and reduction)

Add inhibitors

-- Slow oxidation/reduction reactions by removing reactants(e.g., remove O2 gas by reacting it w/an inhibitor).

-- Slow oxidation reaction by attaching species to

the surface (e.g., paint it!).

Corrosion Prevention Environment

Metal (e.g., Al,stainless steel)

Metal oxide

Cathodic (or sacrificial) protection

-- Attach a more anodic material to the one to be protected.


19/21

Corrosion Prevention Anticorrosion TreatmentsAnticorrosion Treatments

METAL coatings:

via ELECTROLYSIS: Protective films of metal using the coating asthe anode and the part being coated as the cathode,employing sulphates or cyanides as an electrolyte. The substance

is broken down by applying an electrical voltage over acontrolled period at a controlled current, giving us the desired

layer. Steel parts should be covered with 3-4 alternating layers ofCu, Ni, Cr.

NICKEL PLATING.

CHROME PLATING.

COPPER PLATING.

via IMMERSION IN MOLTEN METAL: Immersing the piece in a bathof molten protective metal.

GALVANISING. Pipes/tubes.

TINNING. Tinplate.


20/21

via METALLISING: Using a legalising spray gun to coat the

piece in molten metal.

Zn,P, Al, Stainless steel.

via CASE HARDENING: Methods in which the piece to becoated is covered in powdered Zn and Naphthalene / Cr,

alumina / Aluminium / Si , heated up to set temperature for aspecific time to create a coating on the piece.

SHERARDIZING.

CHROMIZING.

CALORIZING.

SILICATION.

via PLATING: Superimposing plates of metal on one or bothsides of the metal to be protected..

Cu, brass, Ni, Cupro-nickel, Stainless steel.


21/21

NON METALLIC:

inorganic:

Enamel. Via thermo chemical treatment.

PHOSPHATING (immersing the steel pieces in an aqueoussolution of metal phosphate at 100C).

ANODISING (forcing aluminium to oxidise electrolytic allyin order to strengthen the layer of oxide, giving it greater

protection), BLUING (treatment applied to steels toprotect them against oxidation, producing a darkprotective layer of magnetite. Heated up to 200C andthen tempered in oil).

organic:

Paints.

CATHODIC PROTECTION:

Steel pipes using Mg or Zn. Ships propellers which are made of Cu

using

electrodes made of Zn & Mg.

c7-corrosion & degradation of materials

Documents