Material corrosion

corrosion

material loss by dissolution

oxidation

formation of nonmetallic scale or film

degradation

term used for corrosion of polymers

polymer degradation

may dissolve when exposed to a liquid solvent or may absorb solvent and swell, EM waves may cause alreration in molecular structures

corrosion of metals

• electrochemical, beginning at the surface

• meaning chemical reactions involving transfer of electrons from one chemical species to another

• oxidation reaction means metals lose or give up electrons

Galvanic series

• represents reactivities of a number of metals and commercial alloys in seawater

• alloys near the top are cathodic and unreactive

• from metallic to oxidised, there is a decrease in free energy

• so al metals occur in nature as compounds

Passivity

• starts due to very thin oxide film on metal surface serving as protective barrier to prevent further corrosion

• some metals appear twice in galvanic series because some normally active metals and alloys lose chemical reacticity and become inert.

active metals

anodic - lower down galvanic series as corrode easilhy

passive metals

higher up galvanic series as they are cathodic. they are more inert so will not corrode

stainless steel passivation

highly resistant to corrosion in wide variety of environments because of oxide layer. But aluminium goes back to active state if in highly active corrosive environment

Environmental effects

fluid velocity, temperature, composition

fluid velocity

increasing velocity increases rate of corrosion due to erosive effects

temperature increase

corrosion rate rises

increasing corrosive species

higher rate of corrosion, but in some cases induces passivity in some metals

cold worked metals

more susceptible to corrosion (metals that are reshaped without heating)

forms of corrosion (8)

uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion-corrosion, stress corrosion

Uniform attack

• electrochemical corrosion

• common type, easy to design around as predictable

• equivalent intensity over whole surface

reduction reaction

cathodic (gains electrons)

galvanic corrosion

• two metals having different compositions are electrically coupled while exposed to electrolyte

• more corrosive one corrodes first, which protects the inert one

electrolyte

liquid which contains anions and cations which is electrically neutral but when electric potential is applied, charged ions move to respective electrodes and electric current can flow

rate of galvanic attack

dependent on the anode-cathode surface areas exposed to electrolyte. i.e. smaller anode will corrode more rapidly than larger.

reduction of galvanic corrosion (4)

• choose two metals close in galvanic series

• use large anode area

• electrically insulate dissimilar metals from eachother

• cathodic protection

cathodic protection

connecting a third anodic metal to the other two

corrosive environments

atmosphere, aqueous solutions, soils, acids, bases, inorganic solvents, molten salts, liquid metals, human body

atmospheric corrosion

accounts for greates losses

why are marine atmospheres corrosive

sodium chloride

freshwater

contains dissolved oxygen which account for corrosion

seawater

presence of sodium which is corrosive

corrosion prevention

selection of materials, changing environment, lowering fluid temperature and flow velocity as well as composition of corrosives, inhibitors, physical barriers

corrosion inhibitors

• added in low concentrations

• to decrease corrosiveness of environment

• some react with environment, others with metal surface to delay reduction reaction

• others form thin protective coating

• used in close systems such as steam boilers

design impact on corrosiveness

design for exclusion of air and easy wash

physical barriers to corrosion

• applied to metal in form of film and coating

• needs high degree of surface adhesion

• must be non reactive in corrosive environment and resistant to mechanical damage

cathodic protection (in depth)

supplying electrons from anode to metal for it to be protected and become more cathodic - reduction reaction

sacrificial anode

oxidised metal that is added to galvanic couple in order to protect cathode (gives electrons to cathode)

cathodic protection (galvanising)

• ZiNC layer applied to surface of steel by HOT DIPPING

• zinc is anodic to steel so will protecti it

• but corrosion will be slow since the anode to cathode surface area ratio is large

cathodic protection (impressed current)

• protects underground equipment

• external power supply used to make metal cathodic (gain electrons)

• connect inert anode to positive teminal so it can give electrons

oxidation

anodic, loss of electrons, corrosive

corrosion of ceramics

• immune to corrosion in most envirionments

• corrosion involves chemical dissolution

• therefore used for liquid container

degradation of polymers (in depth)

• physiochemical reaction

• can deteriorate via swelling or dissolution

• covalent bond rupture is possible due to radiation

swelling and dissolution of polymers

• swelling: liquid or solute diffuses into and is absorbed within the polymer

• small solute molecules fit into and occupy positions among polymer molecules

• so macromolecules are forced apart so that the specimen swells

• material becomes softer and ductile due to weakened bonding forces

• dissolution: polymer is completely soluble in solvent, continuation of swelling

reducing polymer deterioration

• increase molecular weight and degree of crosslinking + crystallinity

• decrease temp

bond rupture

severence or rupture of molecular bonds: scission

• separation of chain segmetns at point of scission and therefore reduction in molecular weight

bond rupture causes

radiation ,chrmical reaction , thermal effects

weathering

• degradation due to outdoor exposure

• water absorption and oxidation