Corrosion

process of returning metals to their natural state - ores; involves oxidation of metal which causes it to lose its structural integrity and attractiveness

oxide coating

most metals develop thin ______ _________ to protect them --- patinas, tarnish, rust

electrochemical process

Corrosion is an ______________ ___________

reduced

pure metal, metallic atoms, elemental metal

oxidized

compounded metal, metallic ions, ceramic material

-dissimilar metal
-differential aeration
-environmental concentration
-bacteria
-stress
-erosion
-temperature effects

What causes corrosion?

environment

the sum of all the factors that affect the corrosion process
-includes fluids that render charge-transfer possible
-makes possible delivery of reactants to corrosion sites and removal of products of corrosion reactions
-provides medium to transport ionic

-pipelines
-underground storage tanks
-industrial plants
-power generation
-transportation
-marine
-waterworks
-petrochemical plants
-pulp and paper
-real estate
-private homes

application of corrosion prevention

-uniform
-pitting
-stress cracking
-intergranular
-crevice attack
-erosion-corrosion

What does corrosion look like

uniform corrosion

deals with aqueous, gaseous, molten materials, salt and liquid metal environments

aqueous corrosion

the corroding material is an electrode in contact with an electrolyte

high-temperature gaseous corrosion

metal is exposed to an oxidizing gas at elevated temperature; occurs by direct reaction with gas; also referred to as tarnishing, scaling, or high-temperature oxidation

molten salt corrosion

caused by the solubility of the metal in the salt, particularly when the metal dissolved in its own chloride

liquid metal corrosion

occur via dissolution, intergranular attack, impurity and interstitial reactions, alloying and compound reduction

atmospheric corrosion

corrosion due to interaction with the terrestrial atmosphere at its characteristic air temperature, humidity, air chemistry and climactic values

galvanic corrosion

occurs when metal or alloy is electrically coupled to another metal or conduction nonmetal in the same electrolyte; driving force is the potential developed between dissimilar metals/materials

stray current

caused by an externally induced electrical current; follows path other than their intended circuit (also interface current); rail transit system, pipeline systems, electric distribution systems

stress-corrosion cracking

material failure that occur by slow environmentally induced crack growth; result of combined and synergistic interaction of mechanical stress and corrosion reactions

microbiologically-influenced corrosion

biofilms that form on the surface of structural metals and alloys immersed in aqueous environments influence the corrosion of metals and alloys; bacteria, algae, and fungi that are present in all natural aqueous environments

localized corrosion

occurs at discrete locations on a material; includes pitting, crevice and filiform types

pitting corrosion

corrosion that forms on an open surface due to passive films that forms on the metal surface that results in accelerated dissolution of the underlying material

crevice corrosion

corrosion that occurs in occluded site, where a wetted metallic surface is in close proximity to another surface

filiform corrosion

occurs on metallic surfaces with organic films that are typically .05 to .1 mm; appearance of fine filaments emanating in random directions form one or more initiation sites; source of initiation is usually defect or mechanical scratch in coating; filifor

intergranular corrosion

an attack on or adjacent to the grain boundaries of a metal or alloy; heat treatment of stainless steel and aluminum alloys accentuates this problem

exfoliation

form of intergranular corrosion; surface grains of a metal is lifted up by force of expanding corrosion products occurring at grain boundaries just below surface; common in aluminum and carbon steel

fretting corrosion

occurs at the interface between contacting, highly loaded metal surfaces when subjected to slight vibratory motions; can be prevented by lubricating contacting surfaces

erosion corrosion

due to the combination of chemical environment and high-fluid velocity surface; due to fast fluid flow past a stationary object such as a ship's propeller churning the ocean; results from wearing away of protective scale or coating on the metal surface; s

flashlight battery

an engineered corrosion reaction; zinc/carbon cell connected with a conductive electrolyte; life depends on current load

flameless ration heater

a water-activated chemical heater used to heat MRE's and contains finely powdered iron, magnesium metal, and table salt

galvanic cell (battery)

Powdered iron and magnesium, in the presence of salt water, form a ____________ that can generate electricity and heat

aqueous corrosion

electrochemical process that occurs at the interface between a material and an aqueous solution; oxidation (metal dissolution or metal oxide formation) and reduction (proton, water, or dissolved oxygen reduction) occur simultaneously; materials may deteri

an anode and a cathode - locations with a difference in electrical potential (voltage) between them; both locations exposed to the same environment; a metallic connection between the two points

conditions for corrosion to occur

anode

where oxidation occurs

cathode

where reduction occurs

metallic path

where electrons migrate from the anode to the cathode

electrolyte

where ion migration occurs

corrosion cell

electrochemical reaction which involves:
- transfer of electrons
- oxidation
- reduction
- migration of ions

coating

protects the metal by imposing a physical barrier between the metal substrate and the environment
- conversion coating
- anodizing
- diffusion coating
- hot dip galvanizing
- electroplating
- paint

corrosion inhibitors

substances that slow or prevent corrosion when added to an environment where metal typically corrodes; chromates - toxic

cathodic protection

insert active metal like Mg connected by wire to tank or pipeline to be protected; better reducing agent than iron (more readily oxidized); Mg anode dissolves and must be replaced, but protects steel in meantime

galvanization

protective coating of zinc over iron or steel (zinc oxidizes and makes tough ceramic coat); aluminum makes own tough oxide coating
-ex. sheet metal, guard rails, duct work, grain bins, cans with acidic foods

sacrificials

purposely using a more reactive metal to protect a less reactive metal -- it reacts first
- ex. hot water heater - Mg, Al, or Zn rod

water heaters

rod made of magnesium or aluminum that's formed around a steel core wire and is screwed into top of tank

electrolysis

means that when two metals are physically connected in water, one will corrode away to protect the other

ores; chemical, electrolytic

metals are in oxidized form in _______; need to be reduced to get pure metal; two types of reduction:
___________ and _________

chemical reduction

oxidized metal is melted or dissolved and exposed to a more reactive element; can be used on metals that are less reactive than carbon - zinc and below

electrolytic reduction

use electricity on molten or dissolved ore; expensive; used on aluminum and metals more reactive than Al