中文摘要：

长输管道腐蚀绝大部分为外部腐蚀，因此准确检测和评价外防腐层的状况，是保证管道安全运行的基础。文中介绍了用多频管中电流法（PCM）对防腐层进行检测的原理，结合兰成渝长输管线现场的应用，验证了该技术的准确性及科学性。同时，对其使用情况进行了总结，并展望PCM在应用中存在的不足之处。

英文摘要：

The long-distance pipeline corrosion is mainly external corrosion. Therefore, it is very important to accurately inspect and evaluate the corrosion protection performance of external protection coatings for the reliable safe operation of the pipelines. The inspection principles of PCM or corrosion protection coatings are described and applications are presented in the Lan- Cheng-Yu oil pipeline to verify the technical accuracy and scientific adaptivness. At the same time, a summary of its use is given and the remaining deficiencies of PCM in future applications are pointed out.