Overview - Degradation of Materials OPTION The 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 Wood The environmental factors that affect degradation in wood are; Biological organisms – fungi and insects Wood is susceptible to attack when the moisture content exceeds 20%
Degradation of Materials Physical and Mechanical effects of degradation in wood Change in cross-sectional dimensions, swelling and shrinkage Strength and stiffness decrease as moisture content increases Durability is affected Coatings can be compromised
Degradation of Materials Plastics It 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 Plastics UV light will weaken certain plastics and produce a chalky faded appearance on the exposed surface
Degradation of Materials Plastics Heat will weaken or melt certain plastics even at relatively low temperatures
Degradation of Materials Plastics Cold can cause some plastics to become brittle and fracture under pressure
Degradation of Materials Plastics Mould can grow on plastics in moist humid conditions
Degradation of Materials Plastics Bio-degradation – the chemical breakdown in the body of synthetic solid phase polymers
Degradation of Materials Metals Most metals corrode because they react with oxygen in the atmosphere, particularly under moist conditions – this is called oxidation
Degradation of Materials Metals Ferrous metals such as steel are particularly susceptible to oxidation and require ongoing maintenance or they will suffer inevitable structural failure
Degradation of Materials Metals Some non-ferrous metals are particularly resistant to corrosion, e.g. Copper
Degradation of Materials Metals Most corrosion of ferrous metals occur by electro-chemical reaction. This is also known as wet corrosion Electro-chemical corrosion can occur when; Two different metals are involved Metals are separated on the Galvanic Table (potential difference exists) The metals are in contact
Degradation of Materials Metals When two dissimilar metals are placed in a jar of electrolyte (sea water), an electric current is produced
Degradation of Materials Metals When two dissimilar metals are placed in a jar of electrolyte (sea water), an electric current is produced
Degradation of Materials
Degradation of Materials Metals Protection and Finishing There are various protection and finishing treatments applied to metals, they include; Design features Anodising of aluminium Protective coating e.g. paint, plastic, metal Electro plating
Degradation of Materials Metals Sacrificial (cathodic) Protection This is where one metal is deliberately sacrificed to protect another
Degradation of Materials Metals Design Features Avoid, or provide extra protection for, stressed parts, Avoid crevices or sumps that retain moisture Reduce Galvanic effect by careful selection of metals or by design detailing Select an appropriate alloy
Degradation of Materials Metals Anodising of Aluminium An electrolytic process that increases the thickness of aluminium's naturally occurring protective oxide film Organic acid electrolytes will produce harder films and can incorporate dyes to give the coating an attractive colour
Degradation of Materials Metals Paint 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 Metals Protective Coating - Plastic A variety of plastic coatings exist, they include; Brush on coating Electrostatic spraying Hot dipping in fluidised tank
Degradation of Materials Metals Protective Coating - Metal Metal coatings give the best protection – they include; Hot dipping Powder cementation Metal spraying Metal cladding Electro-plating
Degradation of Materials Metals Protective Coating – Electro-plating Uses the chemical effect of an electric current to provide a decorative and/or protective metal coating to another metal object
Degradation of Materials Metals The Effect of Corrosion on Mechanical & Physical Properties Reduction of metal thickness leading to loss of strength or complete structural failure Localised corrosion leading to “crack” like structure. Produces a disproportionate weakening in comparison to the amount of metal lost Fatalities and injuries from structural failure, e.g. bridges, buildings, or aircraft Damage to valves or pumps due to solid corrosion products
Degradation of Materials Metals Environmental Considerations Contamination of fluids/foodstuffs in pipes and containers Leakage of potentially harmful pollutants and toxins into the environment Increased 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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