1. Corrosion & Corrosion Protection

2. Introduction
Metallic bond – within the material metal ions are completely surrounded by like ions
This is not the case at the surface – causes a surface instability – corrosion – the deterioration of metals resulting from this instability

3. Type 1 - Dry
Direct chemical combination – Metals combine directly with gases such as oxygen, chlorine, sulphur gases & carbon dioxide
In an environment containing oxygen – covalent bonds are formed between metals & oxygen at the surface
Once exposed to air oxidation starts – slow process at normal temperatures – rate of corrosion increases with increased temperature

4. Cont’d
Rate restricted by the difficulty in the gas reaching the underlying metal atoms
Rate of oxidation reduces with time - & destruction ceases
Dry atmospheres allow steel components to last for many years
High temperature allows oxide layer to grow rapidly & thicker films tend to crack exposing new metal

5. Free Energy The free energy of the system is reduced by oxidation
Therefore as a consequence almost all metals should revert to their oxides, especially at high temperature
Oxidation is a diffusion process
M + O MO
Metal Oxygen Oxide

6. Type 2 - Wet
Electrochemical corrosion – oxygen may assist the process – results mainly from a tendency of metals to ionise (dissolve) when placed in water or an aqueous environment

7. Cont’d
Ionisation – interaction between surface atoms of a metal & ions in the water – electrons on the metal cause it to become negatively charged – this charge increases as the metal dissolves – reaches electrode potential – charge sufficient to prevent further positive ions leaving the metal & ionisation corrosion ceases
Further corrosion will only take place if the negative charge on the metal is reduced

8. Table of Electrode Potentials
Metal Electrode Potential (V)
Magnesium (Anodic – more basic)
Aluminium CORRODED END
Zinc
Chromium
Iron
Nickel
Tin
Lead
Hydrogen (REF)
Copper
Silver PROTECTED END
Gold (Cathodic – more noble)

9. Cont’d
Electrochemical Corrosion:
Anode
Cathode
Electrolyte
Oxygen

10. Electrolytic Corrosion
Anode Fe
Cathode Cu
Electrolyte
At the anode: M M+ + e-
Surface metal atom
Positive metal ion
Electron remains on metal

11. Cont’d 1.) Acid solution (pH<7) 2H+ + 2e- H2
At the cathode: Reaction depends upon the ions present in the solution & pH
2H+ + 2e H2
In solution
From metal
Hydrogen gas

12. Cont’d 2.) Neutral or alkaline (pH7) 2H2O + O2 + 4e- 4OH-
From cathode
Hydroxyl ions

13. Different Metals in Contact
Corrosion occurs due to differences in electrode potential
Electrons flow from iron to copper (current in reverse direction) – negative charge on the iron is partly reduced allowing corrosion and the iron is termed the anode – the copper does not corrode and is termed cathode

14. Common Corrosion Problems
Metals in contact Corrosion Rectify
Galvanised tank Zinc corrodes Sacrificial anode
& copper pipes or plastic tank
Copper ball-cock Solder corrodes Plastic ball
Soldered to brass in damp atmospheres
Copper flashing Steel corrodes Copper nails
secured by steel nails
Steel radiators with Steel corrodes Corrosion inhibitors
copper pipes

15. Cont’d
I.E. in electrolytic corrosion the metal with the higher position in the table will corrode
The situation is made worse if:
a.) anode area is small compared to cathode
b.) salts are present (additional ions)
c.) temperature is raised

16. Electrolytic Corrosion within a Single Piece of Metal
Variations within the metal structure can result in different electrode potentials at different points on the surface
Steel corrosion is accelerated by the presence of salts – increases the conductivity of the electrolyte - aids the flow of ions in solution

17. Surface Defect
Electrolyte
METAL
Cathode
Anode

18. Cont’d Cause Anode Examples Rectify
Grain structure Grain boundary Steel/damp Isolate steel
Concentration Low Soil types Protective coatings
variations in concentration
electrolyte areas
Differential Oxygen remote Underground Protective coatings
aeration areas steel pipes
Stressed areas Most heavily Steel rivets Protect from
stressed area or nails dampness

19. Concentration Cells
Preferential corrosion due to variations in the electrolyte – the reaction removes electrons & these are supplied by adjacent areas which are deficient in oxygen – these areas act as the anode & hence corrode
Riveted or bolted connections – corrosion occurs in oxygen poor areas
Waterline corrosion of steel sheet piles in stagnant water – oxygen rich at the surface with oxygen deficient layers becoming anodic – corrosion occurs

20. Pitting Corrosion
Similar mechanism – pitting where the oxygen poor region at the bottom of the pit is anodic – pit tends to deepen leading to premature failure due to fatigue or brittle fracture

21. Pit in Metal Plate
METAL
Cathodic
Anodic
Oxygen poor at
base of crack

22. Corrosion Examples
Copper roofing – green patina of copper carbonates & hydroxides – affords good protection except when exposed to sulphurous combustion products or organic acids (lichen growth)

23. Copper Pipes
Pitting corrosion if any carbon film is left on inner surface (extrusion manufacturing)
Copper solution will attack aluminium – copper roof or flashings – run off must be considered & must not be allowed to flow over any aluminium components

24. Aluminium
Most problems caused by bimetallic corrosion – copper or cadmium alloys of aluminium

25. Steel in Concrete
Symptoms - visible cracking & spalling with or without staining – can be from other sources
Some cases – de-lamination occurs - not all cracking is due to corrosion of reinforcement – may be due to excessive structural loading, shrinkage or thermal effects
Factors involved – cover – permeability – level of chlorides

26. Reinforcement Corrosion
Corrosion of steel reinforcement leads to expansion as corrosion products develop.
Expansion leads to tension between the reinforcement & the concrete cover.
Spalling of concrete cover.

27. Timber Can corrode metals (fastenings) – natural acids (acetic, etc.)
Timber preservatives can also cause corrosion particularly when zinc galvanised plates, nails, hinges, etc. – more likely where condensation or high humidity occur

28. Stainless Steels
Produced by alloying steel with chromium (>10%) – coherent – self healing oxide film form on the surface
Virtually corrosion free, except in the presence of chlorides & reducing environments – pitting corrosion, cavitation (associated with components driven at high velocities through a fluid – propellers, impellers etc.), erosion (mechanical effects)

29. Cont’d
Austenitic stainless steel – includes nickel – best corrosion resistance – non magnetic - difficult to weld
Ferritic stainless steel – low carbon steel with 13% chromium – used for steel sinks, steel balustrades – magnetic
Martensitic stainless steel - higher carbon content steel with 13% chromium – tools & cutlery

30. Weld Decay
Caused by poor welding technique – HAZ & formation of carbides

31. Corrosion Control Must be addressed at the design stage
Understand the exposure (environment) – pollution, repeated wetting & drying, humidity, the presence of salts etc.
Design life – maintenance programmes – repair & replacement
Methods of control – longest life for the minimum cost

32. Corrosion Protection
Corrosion occurs at ambient temperatures in the presence of moisture
Design to protect from exposure & if wetted aid drying process – run off surfaces – laps in joints arranged to avoid channels for water – sealant used in joints
Previously rusted steel left on site will be liable to further corrosion as rust pits are difficult to remove even after aggressive cleaning – protect site steel with primer – prevent initial corrosion

33. Isolation Methods
Application of protective coatings to specially prepared surfaces
Metallic coatings:
protective barrier - steel protected by nickel or chromium
Anodic coatings – sacrificial protection – steel protected by zinc, cadmium or aluminium

34. Cont’d
Organic coatings – protective barrier - paints, pitch & tar – usually applied over a metallic primer
Application must be to clean & dry surfaces that have been properly prepared

35. Cathodic Protection This can be achieved in two ways:
1.) Sacrificial Anode
2.) Impressed current
It has been shown that a combination of cathodic protection & coating is the most economical means of protecting steel structures

36. Sacrificial Anode Use of sacrificial anodes – Zinc, lead, etc.
Used on small structures
Anodes welded or bolted to fixtures
Need regular checks for wastage
Recently aluminium oxides have become more popular due to better performance to weight

37. Example Sacrificial Anodes

38. Cont’d Pipelines buried underground are protected in this way
Method relies on conductive pathways through the soil
Spacing coating design and coupling are important factors
Anodes are fixed in bands weighing 300 to 400kg at intervals of about 150m

39. Impressed Current (ICCP)
Involves the use of an external power source – metal to be protected is made cathodic to its surroundings – inert anodes used which are virtually non-consumable – insulated from structure
Early anodes made from scrap steel but most modern ICCP systems use lead silver alloy, titanium or niobium

40. Transformer Rectifier Unit

41. Cont’d
Has been used in the protection of steel reinforcement in concrete
The use of modern electronics makes the system self regulating
Very costly to run – mainly used in marine applications – oil rigs – large anodes placed on sea bed approximately 100m away

42. Impressed Current System

43. Typical Alternatives for a Buried PipelineMg
Magnesium Anode
Impressed current