Overview - Degradation of Materials OPTIONThe student will learn about…The effect of environmental conditions on the mechanical and physical properties of materials The student will be able to…Describe the conditions that cause the physical, chemical and biological degradation of materials.Describe how materials degrade in certain conditions and how materials are altered by degradation.
Degradation of Materials Corrosion is the deterioration of a material as a result of a reaction with its environment, especially with oxygen (oxidation).
Finishing of Materials WoodThe environmental factors that affect degradation in wood are;Biological organisms – fungi and insectsWood is susceptible to attack when the moisture content exceeds 20%
Degradation of Materials Physical and Mechanical effects of degradation in woodChange in cross-sectional dimensions, swelling and shrinkageStrength and stiffness decrease as moisture content increasesDurability is affectedCoatings can be compromised
Degradation of Materials PlasticsIt is widely accepted that plastics do not corrode however micro organisms which can decompose low density polyethylene do exist
Degradation of Materials
Degradation of Materials PlasticsUV light will weaken certain plastics and produce a chalky faded appearance on the exposed surface
Degradation of Materials PlasticsHeat will weaken or melt certain plastics even at relatively low temperatures
Degradation of Materials PlasticsCold can cause some plastics to become brittle and fracture under pressure
Degradation of Materials PlasticsMould can grow on plastics in moist humid conditions
Degradation of Materials PlasticsBio-degradation – the chemical breakdown in the body of synthetic solid phase polymers
Degradation of Materials MetalsMost metals corrode because they react with oxygen in the atmosphere, particularly under moist conditions – this is called oxidation
Degradation of Materials MetalsFerrous metals such as steel are particularly susceptible to oxidation and require ongoing maintenance or they will suffer inevitable structural failure
Degradation of Materials MetalsSome non-ferrous metals are particularly resistant to corrosion, e.g. Copper
Degradation of Materials MetalsMost corrosion of ferrous metals occur by electro-chemical reaction. This is also known as wet corrosionElectro-chemical corrosion can occur when;Two different metals are involvedMetals are separated on the Galvanic Table (potential difference exists)The metals are in contact
Degradation of Materials MetalsWhen two dissimilar metals are placed in a jar of electrolyte (sea water), an electric current is produced
Degradation of Materials MetalsWhen two dissimilar metals are placed in a jar of electrolyte (sea water), an electric current is produced
Degradation of Materials
Degradation of Materials MetalsProtection and FinishingThere are various protection and finishing treatments applied to metals, they include;Design featuresAnodising of aluminiumProtective coating e.g. paint, plastic, metalElectro plating
Degradation of Materials MetalsSacrificial (cathodic) ProtectionThis is where one metal is deliberately sacrificed to protect another
Degradation of Materials MetalsDesign FeaturesAvoid, or provide extra protection for, stressed parts,Avoid crevices or sumps that retain moistureReduce Galvanic effect by careful selection of metals or by design detailingSelect an appropriate alloy
Degradation of Materials MetalsAnodising of AluminiumAn electrolytic process that increases the thickness of aluminium's naturally occurring protective oxide filmOrganic acid electrolytes will produce harder films and can incorporate dyes to give the coating an attractive colour
Degradation of Materials MetalsPaint is widely used particularly to protect steel. It is not effective over time and under certain conditions and must be renewed regularly – often at considerable expense
Degradation of Materials MetalsProtective Coating - PlasticA variety of plastic coatings exist, they include;Brush on coatingElectrostatic sprayingHot dipping in fluidised tank
Degradation of Materials MetalsProtective Coating - MetalMetal coatings give the best protection – they include;Hot dippingPowder cementationMetal sprayingMetal claddingElectro-plating
Degradation of Materials MetalsProtective Coating – Electro-platingUses the chemical effect of an electric current to provide a decorative and/or protective metal coating to another metal object
Degradation of Materials MetalsThe Effect of Corrosion on Mechanical & Physical PropertiesReduction of metal thickness leading to loss of strength or complete structural failureLocalised corrosion leading to “crack” like structure. Produces a disproportionate weakening in comparison to the amount of metal lostFatalities and injuries from structural failure, e.g. bridges, buildings, or aircraftDamage to valves or pumps due to solid corrosion products
Degradation of Materials MetalsEnvironmental ConsiderationsContamination of fluids/foodstuffs in pipes and containersLeakage of potentially harmful pollutants and toxins into the environmentIncreased production/design and ongoing maintenance costs. This results in greater use of scarce resources and the release of harmful CO² gasses into the environment
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