Corrosion Flashcards
define surface corrosion
-> caused by direct or indirect chemical attack
Types of corrosion
- > chemical
- > galvanic
define pitting corrosion
localised form of corrosion
- > hard to see becomes stress raiser
- > caused by chemicals or mechanical damage
define Crevice corrosion
- > most common
- > caused by stagnant solution (micro-environmental)
- > occurs in fastener heads, thread etc
- > caused by absorption of oxygen into the liquid. surface more O2 creates cathode.
How is crevice corrosion detected
detected by pillowing or opened joints
define intergranular corrosion
also known as intercrystaline corrosion
only seen on aluminium alloy or stainless steel
intergranular corrosion caused and detect by
caused due to incorrect manufacture or heat treatment
- > grain growth in the boundary area creates dissimilar metal corrosion
- > only detected with X-Rays or when item falls off
define exfoliation corrosion
AKA laminar corrosion
- > only in extruded or rolled metals
- > mostly aluminium alloys
- > follows the grain
Define Fretting Corrosion
- > corrosion takes place between two loaded surfaces
- > surfaces are moving
detection and treatment of Fretting corrosion
- > recognised by cocoa powder
- > prevented by sealant
Define Stress corrosion
- > only occurs with metal under tensile stress
- > the stress can be natural or created
- > cracks develop and lengthen quickly
Define Filiform Corrosion
- > only occurs under grease, oil or varnish
- > can occur due to incorrect painting
Define Mercury contamination
-> only occurs with mercury spillage
how is mercury contamination removed
- > vacuum cleaner with mercury trap
- > syringe
- > damaged area cut out and repaired
Define Microbial corrosion
-> hormoconis resinae ( cladosporium resinae)
Define Hydrogen embrittlement
- > occurs in ferrous, aluminium and titanium alloys
- > must have sustained loads (UTS)
- > hydrogen must be present in the metal
Define anodising
- > protection from corrosion
- > electrolyte bath
- > oxygen combines with the component corroding it a little but ultimately protecting it
Define Chromic acid anodising
- > good corrosion resistance
- > coating is thin and relatively soft
Define sulphuric acid anodising
-> various colours
Define Chemical treatment
- > immersion or painting of a component with a specific chemical for a specific time
- > one type Alocrom
- > different chemicals produce different colours
Define Pasivation
chemically passive layer
what are the different types of metal coating
- > cladding
- > metal spray
- > electroplating
Define cladding
-> sheet aluminium has 99.9% pure aluminium layer rolled onto both sides
Define metal spray
-> a metallic coating of aluminium, zinc, tin, copper, lead, brass and bronze
-> two types , metallising = zinc or aluminium
Aluminising = aluminium alloys
Define electroplating
- > carried out in a bath with an electrolyte, component and plating material
- > coating of codmium or zinc = less noble
- > coating of nickel or chromium = more noble
Define partial plating
- > area not being plated has a wax applied onto it
- > and lacquer
- > masked with lead foil
