Buried metal pipeline corrosion is the main form of electrochemical corrosion, caused by the main reason for the electrochemical corrosion: metal pipe's electrochemical uneven of (composition, crystallization methods, organization and oxide skin, etc.) and the surrounding soil medium physical and chemical properties (ion concentration, water content, oxygen content, etc.) of different, and the formation of numerous microscopic galvanic corrosion, corrosion occurs at the site of primary battery anode. The corrosion of metals in soil and fresh water is the dominant factor in the depolarization of oxygen. The corrosion of metals can be increased when there are sulfur iron bacteria in the soil. According to the practical experience at home and abroad, the external anti-corrosion insulation coating and electrochemical protection is to inhibit or delay the corrosion rate of the steel pipe is a economical and effective method, the two complement each other, use at the same time, in order to achieve good results.
(1) in 1977 in China Northwest Municipal Engineering Design Institute in the design of Ningxia Rujigou coal mine, high-voltage 45km long distance water transfer project using Daqing new No.1 asphalt anticorrosion measures, 2mm thick asphalt layer, two layers of glass cloth, outsourcing a layer of plastic sheeting, total thickness > 6mm. At the same time, we use the electric chemical physics joint anti-corrosion measures, to supplement the single asphalt coating insufficient. The method of electrochemical corrosion protection is a cathodic protection station set up at the first pressure pumping station of the water transmission pipe. SF - 2 constant potential instrument for protection and current, anode ground bed with a spacing of 35m, a total of 10 root diameter 100mm 2500mm long old steel composition, in order to increase the conductivity of the bed, steel pipe around 1m backfill coke powder, and method of water wet guarantee bed resistance below 1.0 Omega, anode ground bed and dry pipe confluence point distance of 500m. In order to regularly check the protection effect, a protective potential test pile and a corrosion proof check sheet are arranged at each 1km at intervals along the pipeline. After operation, the effect is good.
(2) in 1983, Shanghai Municipal Engineering Design Institute in the design of Shanghai Baosteel raw water from Yangtze River Engineering, 2 the 14 km long with a diameter of 1200mm water pipe the second cloth oil epoxy coal tar coating anticorrosion, preservative used in cement mortar coating. At the same time, we use sacrificial anode electrochemical protection.
In 1985, Shanghai Municipal Engineering Design in the design of Shanghai Huangpu River upstream water diversion project, 20.2km long steel pipe is used, with the maximum diameter of the 3000 mm in diameter, on how to reduce the corrosion of steel tube, prolong the service life, in-depth study. First, the soil along the line test, belong to the type of strong corrosion. Determine the use of external anti-corrosion insulation coating and electrochemical protection. For the choice of external anticorrosive coating, according to introduce the relevant information, same anticorrosion insulation coating such as the use of manual rust, its life for 2 years and 3 months; such as the use of acid pickling is able to maintain the 9 years and 6 months, so I decided to the pickling and passivation process. There are many kinds of coatings, which are screened by many experiments, and finally determined by using super thick slurry type epoxy coal tar pitch anticorrosive coating. Concrete construction and straight sections were applied with three layers, the first layer to 702 epoxy zinc rich primer, the second layer 842 epoxy primer, the third layer is 546 ultra thick epoxy asphalt paint, the total thickness of the film 290u. For inner anticorrosion coating, the study to the cement mortar lining is the most economical and efficient, the anticorrosion mechanism, in addition to play the role of isolation, but also the use of mortar in the solution, the analysis of Ca222 + ions (pH 12) on the surface of a steel pipe to form a dense layer of passivation film, thereby resulting in the inhibition of water erosion. For electrochemical protection, due to the buried steel pipeline segment length, at about 3~6km, along with multiple and other metal pipe and wire cross or parallel laying, mutual interference effect, and impossible to protect United. Therefore, only by sacrificial anode protection. This is by using different metal potential difference protected against corrosion current, in the underground metal body access potential than the low metal as the anode the earth battery was formed, at the expense of the anode to prevent corrosion of underground metal. At present, the sacrificial anode series of buried metal pipeline in China are: magnesium, aluminum, zinc three alloy, by the comprehensive comparison, the design of magnesium alloy sacrificial anode. The construction surveying home bridge water pipe, pipeline protection potential were about 1.1V, more than minimum protection potential 0.85v. It is proved that the protection effect is good.
The Huangpu River pipe segment of electrochemical protection, in view of the fact that the protection tube most section of the under the riverbed, in order to set up the convenience of anode, prolongs the replacing period of anode, also in order to adjust the parameters of the protection, providing long distance protection, the impressed current protection method.
