1 Corrosion-resisting Coating
Comprehensive analysis and study on corrosion-resisting coating shall be made based on its design service life, environmental media, construction conditions, economy and other factors so as to determine the type, thickness and surface treatment grade of paint. Aluminum (zinc) spraying is a long-acting corrosion-resisting technique. Aluminum (zinc) is sprayed onto surface of steel base and creates a electro-chemical corrosion resistant coating that can remarkably improve corrosion protection effect. Thanks to its favorable binding force and above-noted features, aluminum (zinc) spraying has been widely applied on steel structure surfaces in damp environment or areas with severe conditions such as ebb and flow zone (between river and ocean) and other environments with high content of chloride ion.
Surface treatment shall be performed before coating, and the quality of surface treatment has extremely large impact on coating quality. Different paints correspond to different steel rust removal quality requirements. Generally speaking, conventional oily paint with more favorable wetting property and permeability has slightly lower requirement on rust removal quality, while high performance paint such as zinc-rich paint requires high-quality rust removal.
Paint application shall be performed at a relative humidity of less than 85% and a temperature of more than 5℃; open-air construction is not allowable on rainy, snowy and foggy days, or coating adhesion would be adversely affected. Be sure to measure steel sheet temperature and relative humidity before coating. Only when surface temperature of steel sheet is more than 3℃ higher than dew point, can the construction be carried out.
Coating method shall be selected based on physical property of paint, construction conditions, requirements on coating and the condition of structure to be coated. Coating methods include brush coating, roller coating and high pressure airless spraying etc., and the coating could also be performed according to instructions of manufacturer. Metal spraying is classified into crucible spraying, powder spraying coating, wire spraying and plasma spraying depending on the type of spray gun used. Crucible spraying has been eliminated due to its inconvenience of use as the paint should be molten in crucible and then be sprayed out using compressed air. With respect to powder spraying, metal powder is fed into powder spray gun using powder feeder, molten with fuel gas and then sprayed out using compressed air. Since metal powder is hard to obtain, this method is seldom used at present. As the substitute for metal powder spraying, wire spraying is a common method in case of both arc melting and fuel gas melting. However, this method is applicable only to metals with relatively low melting point that can be turned into wires. The spraying of refractory metals and their oxides, carbides, silicides and borides etc. could employ plasma spraying method by which high-temperature plasma flame would be produced.
Coating inspection standard:
(1) Corrosion protection by coating
① Coating appearance standard: Coating appearance shall be uniform, flat and lustrous; defects such as lifting, crack and pinhole stripping etc. is not allowable. Slight local sag, brush mark, wrinkle and small quantity of particle dust that have no impact on performance are allowable.
② Coating thickness standard: The coating thickness measured with paint film thickness gauge must be able to meet design specified standard. Criterion of acceptability: coating of which the measured overall average thickness reaches 90% of nominal thickness is deemed qualified; in the calculation of mean value, the measurement point thickness that is 20% more than nominal thickness is calculated as 120% of nominal thickness.
(2) Corrosion protection by sprayed metal coating
① Coating appearance standard: Coating appearance shall be uniform and free of peeling, swelling, crack and chipping.
② Coating thickness standard: coating thickness must be able to meet design specified standard; the arithmetic mean value of thicknesses measured at measuring points using magnetic thickness tester shall be taken as the coating thickness at studied position; the testing position and quantity and the criterion of acceptability are determined based on design requirements.
③ Coating adhesion inspection standard: scribe grids in parallel with sharp cutter on coating with a spacing of 3~5mm, cohere with adhesive tape tightly and then pull it upwards vertically; the coating that does not fall off and show the base is deemed qualified.5~10 points shall be sampled randomly and uniformly on each piece of pile.
2 Corrosion Protection by Concrete Covering
Concrete or mortar is coated on the surface of steel. The large quantity of slaked lime generated through hydration or hydroclastic split of cement ensures the basicity of moisture environment of mortar, thus creating iron rust preventing mechanism.
Since corrosion protection by cathodic protection responds only to the section submerged in water, the top covers for revetment and quay wall of steel sheet pile are usually fabricated of concrete. When using this method, be sure to maintain adequate coverage thickness so as to prevent cracking and peeling.
3 Select Corrosion Resistant Type of Steel
The first thing designers have to think about is the selection of corrosion resistant steel based on the difference of corrosive medium. With progressive development of metallurgical industry, new types of special steels have been continuously developed; accordingly, it is possible to select corrosion resistant steel. For example, 16MnCu, 10CrMoAl and 10CrMoCuSi are used for seawater works; however, technical and economic evaluation shall be performed and other supporting protection measures shall be taken before the use.
4 Cathodic Protection
Cathodic protection is suitable for corrosion protection for steel pile below mean tide level of harbor work. Impressed-current protection system, sacrificial anodic protection system or the combination of these two systems is usually used for cathodic protection.
(1) Sacrificial anodic protection system
Sacrificial anodic protection is applicable for corrosion protection for steel structure below mean tide level of harbor work in sea water or brackish water of which the receptivity is less than 500Ω•cm (the use in sea water is recommended).Material of sacrificial anode shall have adequately negative electrode potential and be able to maintain surface activity during the use and to ensure uniform dissolution, the easiness of peeling of corrosion products, high capacitance, high current efficiency, processing and manufacturing easiness, sufficient source and low cost etc.
Construction of sacrificial anodic protection system:
① Sacrificial anode must be firmly mounted on steel structure to be protected and connected thereto in a short-circuit manner through welding (recommended) or fittings or cables;
② The size, weight, surface condition and iron core of sacrificial anode shall be inspected before installation, and the working surface shall not be stained with paint and oil stain; in addition, its mounting position shall adhere to design specification;
③ When sacrificial anode is fixed through underwater welding, welding shall be performed by underwater welding operators who have obtained certificate.
Quality inspection of sacrificial anodic protection system:
① Sacrificial anode shall be inspected according to design specification at delivery, and the manufacturer shall provide quality certificates regarding chemical composition, current efficiency, operating potential and open circuit potential in sea water, solubility property, dimensions, weight, and the contact resistance between anode body and iron core etc.
② Constructor shall perform construction according to design requirements and check the installation quality in a timely manner so as to ensure the reliable short-circuit connection between sacrificial anode and protected steel structure. When underwater welding is employed, the weld shall, to the greatest extent, be inspected through underwater photography or television, and the quantity of sacrificial anodes of which the weld is inspected shall not be less than 5%~10% of total number of sacrificial anodes. When nonconforming weld is detected, overall weld inspection shall be performed, and the nonconforming ones shall be repaired in a timely manner.
③ After the installation of sacrificial anode is over, constructor shall submit cathodic protection completion drawing indicating the actual number of anodes and their positions while measuring the potential at surface of protected steel structure. In case the potential fails to meet design requirements, remedial measures such as re-welding, replacement or supplement of sacrificial anode shall be taken in a timely manner.
(2) Impressed-current protection system
Impressed-current protection system normally includes auxiliary anode, direct supply (potentiostat or rectifier), reference electrode, testing equipment and cables.
Construction and quality inspection of impressed current cathodic protection system:
① When making electrical connection using welded steel reinforcement, be sure to set up marks and perform close inspection before concrete pouring; sold skip is not allowable. Connection points of reinforcement welding or cable connection in atmospheric zone or splash zone shall be protected through sealing.
② Auxiliary anode and its shielding board (tube) shall be installed according to the requirements of construction drawings. Metallic short circuit is not allowable between auxiliary anode and protected steel structure. There should be adequate allowance of the length of auxiliary anode cable under water.
③ Reference electrode shall be able to pass the inspection before installation and be operated according to the requirements of construction drawing. There should be certain allowance of the length of reference electrode cable under water. Ground point of reference electrode shall not be the same with or close to anode ground point.
④ DC power supply shall be mounted in a well-ventilated area where it is convenient to remove dust. DC power supply mounted outdoors in a dispersed manner shall be provided with well-ventilated drip-proof metal enclosure. Metal enclosure of DC power supply shall be grounded and its grounding resistance shall be less than 4Ω.When positive and negative electrodes of DC power supply are connected to corresponding anode cathode cables, polarity reversed is strictly prohibited.
⑤ Cable shall be arranged in steel pipe, PVC pipe and covered cable trench (bracket) and shall in no case be exposed to sun and the corrosion by strongly corrosive substances.
⑥ In case of segmented connection, cables shall be furnished with favorable sealing means and shall not be exposed to surrounding environment;
If necessary, they could be placed in junction box (case), and their anode connection shall in no case be in contact with metal enclosure of junction case (box).Try not to damage the sheath during the laying of cable; cable shall be repaired in case of partial damage, and be placed if serious damage occurs.
⑦ After the construction of cathodic protection system is completed, overall inspection shall be carried out for construction quality according to relevant norms and design requirements, and records shall be made.
5 Corrosion Protection of Cladding Layer
To realize long-acting corrosion protection, steel sheet pile manufacturer may cover its products with special HMWHDPE or polyurethane formic acid ethyl ester synthetic rubber in plant. Since the protecting material used is a kind of synthetic resin, electrical insulation is excellent. Furthermore, as corrosion control can adequately protect piles against high cohesive strength and stock-proof strength of the sea and other severe environments, it is ideal.
As is shown in Fig. 5.7.7-1, steel sheet pile from which the rust has been removed is applied with a layer of special surface conditioning agent (primer) in order to improve corrosion protection and bonding effect, and thereon, HMWHDPE or polyurethane formic acid ethyl ester synthetic rubber (two layers in case of polyethylene; the under-layer is bonding agent as shown in figure) is applied.
Try not to damage protecting layer during construction of steel sheet pile with corrosion protection through cladding layer. It should be particularly noted that serious contact with guide frame and other materials during hoisting and driving may result in peeling, so crude assembly must be avoided.
China has not yet issued standard for inspection of quality of corrosion protection by cladding layer. The following requirements could be followed when specific design specification is unavailable:
(1) Cladding layer appearance standard: the appearance of cladding layer shall be flat, smooth and free of holes and exposed organic material. The number of bubbles with an area of less than 0.5 cm2 shall not exceed 3 per square meter. Finishing layers should be provided with uniform paint film and free of serious sag and cockles.
(2) Cladding layer thickness standard: overall thickness of cladding layer could be measured with magnetic thickness tester. The testing position, number of test points and approval standard could be determined according to design requirements.
(3) Cladding layer withstand voltage test standard: withstand voltage test is conducted using special electrical equipment. Set test voltage according to design requirements and move the probe along the surface of tested cladding layer; if an alarm is given, it means that electrical spark breakdown occurs at his point, indicating that the cladding layer has defects such as thickness insufficiency, bubble, or failure of paint to fully percolate organic material. Carry out the test on a randomly selected surface area that accounts for 50% of total surface area along pile axis. The test is deemed to have been passed the number of alarm points does not exceed 5.