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