The relations between Pitting Corrosion and Cathodic Protection in Cang——Lin Pipeline

The Cang--Lin Pipeline was insulated with the ordinary grade of bituminous coating and glass cloth, and protected completely by cathodic protection. Since this pipeline was speedily constructed with its construction quality poorly controlled, pitting corrosion and pipe leaks occurred frequently. A frequency curve of pitting corrosion and pipe leak is drawn based on the statistics of leakage over several years and the reasons for the three leakage peaks are discussed: the first peak is the result of defects of raw material and welding, and the second and third peaks are caused by pitting corrosion. The last two peaks are further analysed taking account of the condition of cathodic protection along pipeline? Because the cathodic protection stations were put into service until pitting corrosion occurred and no repair was made for the corroded pipeline, a great number of dead areas of cathodic protection occurred within the coating. Pipeline corrosion in these dead areas continued and resulted in a leakage peak. This article suggests to carry out one major pipeline repair every year, and reminds people to pay great attention to cathodic protection and make use of it as soon as possible.