Corrosion Prevention

• Materials - Noble or close on series
• Where dissimilar metals in contact, more anodic metal should constitute larger surface area
• Protective coatings
• Environment - pH, de-aeration
• Cathodic Protection - Sacrificial anode
• Detail design - eg, avoidance of crevices, draining
• Vehicles: ventilation to minimise moisture

Consequences of Corrosion

• General thinning (wastage)
1. Material corroded fairly uniformly over wide area, and where the flaky and porous corrosion products no barrier to further corrosion
• Passive film formation (beneficial)
1. Corrosion products form durable & impervious barrier which slows down / halts more corrosion
• Oxidation (mainly high temperature ‘dry’)
1. Consequence: scale formation which causes eg blockages in pipe work
• Pitting
1. Material selectively attacked to form spherical pits

• Cracking
1. Selective attack leads to formation of deep, sharp-fronted cracks

Corrosion

• Deterioration of materials through chemical or electro-chemical interaction with environment
• Attack on metals by agents such as polluted air; sea water (sodium chloride), leading to formation of rust
• Most corrosion ‘wet’
• Lecture approach mainly not via chemical reactions
1. Not a unifying theme for explanations of different corrosion modes
2. Personal sensitivities



• Two different metals electrically in contact immersed in electrolyte
1. Creates galvanic couple where more active metal corrodes (anode) at accelerated rate and more noble metal (cathode) corrodes at a retarded rate (or zero)
• Electrical cells also created by eg:
1. Air bubbles / water droplets
2. Differential stresses in same metal causing local galvanic couple (stress corrosion cracking)

Corrosion

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Formation of an oxide of iron due to oxidation of the iron atoms in solid solution is a well-known example of electrochemical corrosion, commonly known as rusting. This type of damage typically produces oxide(s) and/or salt(s) of the original metal. Corrosion can also refer to other materials than metals, such as ceramics or polymers, although in this context, the term degradation is more common.

In other words, corrosion is the wearing away of metals due to a chemical reaction.

Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate-conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.

Brittle - Ductile - Creep Example

BOILER SUPERHEATER FAILURE EXAMPLE
from left to right, the first one will be Ductile Failure, the one in the middle will be Brittle Failure, and the one on the right will be Creep Failure.

Preventing Creep Failures

• Design stresses and temperatures based on creep test data
• Selection of materials
1. Good: eg, 1% or more chromium; 0.5% molybdenum steel
• Minimisation of thermal expansion stresses

Creep Failures

• Excessive deformation
1. Gas turbine blades
2. Lead on roofs
3. Suspension bridge cables
• Rupture
1. Trans-granular: deformation
2. Inter-granular: little / no deformation where longer lives; higher temperatures


• Pre-stressed concrete (eg, in fires)
1. Pre-stress eliminated
• Prolonged overheating
1. Just 50ºC above specification: 90% reduction in life
2. 11 years at 500ºC; one hour at 700ºC