Corrosion & Corrosion Protection

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

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

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

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

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

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

Table of Electrode Potentials Metal Electrode Potential (V) Magnesium -2.40 (Anodic – more basic) Aluminium -1.76 CORRODED END Zinc -0.76 Chromium -0.65 Iron -0.44 Nickel -0.23 Tin -0.14 Lead -0.12 Hydrogen (REF) 0.00 Copper +0.34 Silver +0.80 PROTECTED END Gold +1.40 (Cathodic – more noble)

Cont’d Electrochemical Corrosion: Anode Cathode Electrolyte Oxygen

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

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

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

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

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

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

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

Surface Defect Electrolyte METAL Cathode Anode

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

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

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

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

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)

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

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

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

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.

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

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)

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

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

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

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

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

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

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

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

Example Sacrificial Anodes

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

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

Transformer Rectifier Unit

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

Impressed Current System

Typical Alternatives for a Buried Pipeline Mg Magnesium Anode Impressed current