Cathodic Protection in Marine Structures Bird （阴极保护海洋结构的鸟）.pdf

Cathodic Protection in Marine Structures Bird Repair and Maintenance of Marine Structures by Cathodic Protection Alan R. Bird, Michael P Cope† and Anna Rix* † Solomon Corrosion Control Services Pty Ltd, 22 Eskay Road, Oakleigh South, VIC 3167 *Monash University, Department of Civil Engineering, Melbourne, VIC 3000 Abstract Marine structures such as bridges, wharves, jetties and mooring dolphins are required to function in an aggressive environment for many years. Maintenance expenditure is sometimes sporadic and the required service life may be increased as the structure ages, with the result that many marine substructures deteriorate to the extent that major repairs are needed. The technology available for repair and maintenance of reinforced concrete structures has grown in recent years and electrochemical treatments, such as cathodic protection, are now becoming common for marine structures. This paper indicates the range of cathodic protection systems and hardware in current practice, with reference to the treatments applied to port structures in Victoria, Queensland, New South Wales and Tasmania. Their different methods of contract delivery are also indicated. 1. REINFORCED CONCRETE IN MARINE ENVIRONMENTS Steel and concrete have been used effectively together in civil construction for over 100 years. Their combination has been commonly used because of its versatility, durability, fire resistance and low cost. However embedded steel reinforcement frequently corrodes and this corrosion can cause significant deterioration of the structure. 1.1. The Corrosion Process During construction a protective oxide film forms on the surface of reinforcing steel as the cement hydrates. The film is formed due to the high alkalinity (pH 12.6-15.5) of the hydrating cement and it will continue to protect the steel against corrosion while the high alkalinity is retained. Concrete is a microporous material and therefore susceptible to the permeation of gases and diffusion of ions, such as chlorides, in solution. Penetration of chloride ions in the concrete microstructure will occur where elements are exposed to seawater and salt spray. In most cases the laws of diffusion can approximately predict the rate of chloride ingress. Salt water is rapidly absorbed by dry concrete through the initial mechanism of suction. Chloride ions then move through the concrete pores through diffusion. When in sufficient concentration, the presence of chlorides at the steel surface can initiate corrosion. Once the layer is destroyed the chlorides are not consumed in the reaction but act as a catalyst in the ongoing corrosion process. Corrosion is an expansive process because rust products occupy a much larger volume than the original steel. This leads to cracking, delamination and finally spalling of the concrete cover to the steel. Once exposed to the atmosphere, the steel reinforcing will then continue to corrode at an accelerated rate. In low oxygen conditions, such as in buried or submerged areas, the corrosion rate is reduced due to the limited availability of oxygen. Corrosion under these conditions is slower and usually results in a partially soluble and less expansive ‘black rust’. Chloride ingress is the principal cause of extensive and severe corrosion of embedded steel reinforcing in concrete structures at coastal locations. The implementation of remediation measures is usually left until the presence of chlorides is widespread with high concentrations of chlorides at or beyond the level of steel reinforcement. At this stage, cathodic protection is often the most appropriate technique to prevent further deterioration. 1.2 Cathodic Protection of Reinforced Concrete Cathodic protection of metals against corrosion was first demonstrated in the early nineteenth century and it is now used in a wide variety of applications to slow corrosion of metals including pipelines, ships’ hulls, underground storage tanks and offshore structures. Cathodic Protection in Marine Structures Bird Since the 1970s this technology has been applied to effectively stop corrosion of embedded steel reinforcing within concrete structures. The anodes used in cathodic protection systems for buried and immersed concrete structures are well established but the technology of the anodes for atmospherically exposed concrete structures is still evolving. Over the past ten years cathodic protection is being used more frequently as a cost-effective solution to control corrosion of steel reinforcing in atmospherically exposed concrete structures. Cathodic protection controls the corrosion process by altering the thermodynamics and kinetics of the affected steel and is achieved by forming an electrical circuit with an introduced anode using the concrete as an electrolyte and the protected steel as the cathode. Cathodic protection can be driven either by galvanic means or an impressed current. 2.0 CATHODIC PROTECTION SYSTEMS Galvanic and i。