Failure Analysis of 20G Seamless Steel Pipe Perforation Leakage

Literature Overview

The paper by Zhang Qinglian, Li Ming, and Wang Qijiang (Baoshan Iron and Steel Co., Ltd., 2016), published in Physical Testing and Chemical Analysis - Physical Analysis (Vol. 52, No. 4, pp. 265-269), presents a systematic failure analysis of a 20G seamless steel pipe that experienced perforation leakage during service. The investigation combined corrosion environment characterization, material examination, corrosion product analysis, and water quality assessment to identify the root cause of the failure. This case study provides valuable lessons for the selection, protection, and monitoring of low-carbon steel pipes in aggressive aqueous environments.

Failure Mechanism Analysis

The investigation revealed that the perforation leakage was caused by localized corrosion, specifically pitting corrosion, initiated on the internal surface of the pipe due to the aggressive nature of the water medium. The 20G steel, a normalized low-carbon steel with approximately 0.2% carbon content and no significant alloying additions, lacks inherent resistance to localized corrosion attacks. The water quality was characterized as hard with elevated concentrations of chloride ions (Cl-) and fluoride ions (F-), both of which are potent initiators and accelerators of pitting corrosion in carbon steel.

Corrosion Mechanism Breakdown

5W2H Analysis of the Failure

• What: Perforation leakage of a 20G seamless steel pipe.
• Where: Internal surface of the pipe, at locations where localized pitting initiated and propagated through the wall thickness.
• When: During normal service operation, after a period of progressive localized corrosion.
• Who: The pipe was used in a water transport or process application at Baoshan Steel.
• Why: The combination of a non-corrosion-resistant base material and an aggressive water chemistry containing Cl- and F- ions.
• How: Pitting corrosion initiated at surface defects or inclusions, grew through the wall thickness under the sustained attack of aggressive ions, and ultimately resulted in perforation.
• Which countermeasures: Improve water quality treatment and monitoring; substitute with corrosion-resistant Cr-Mo low-alloy steel pipe.

Recommendations and Engineering Implications

The authors recommend two primary corrective measures: first, improving the water quality treatment and implementing continuous monitoring of water chemistry parameters, particularly Cl- and F- concentrations; second, replacing the existing 20G steel pipe with a Cr-Mo low-alloy steel pipe that offers superior resistance to pitting corrosion. From a metallurgical perspective, the Cr-Mo alloying provides a dual benefit: chromium enhances the stability of the passive oxide film and raises the pitting resistance equivalent number (PREN), while molybdenum specifically improves resistance to chloride-induced pitting and crevice corrosion.

Material Selection Guidance

Study Insights and Reflections

This failure analysis case reinforces a fundamental principle in materials engineering: the selection of a pipe material must be based on a thorough understanding of the service environment, not merely on mechanical property requirements. The 20G steel, while perfectly adequate for many pressure vessel and boiler applications, is ill-suited for aqueous environments containing aggressive halide ions. The failure mode—pitting corrosion leading to perforation—is particularly insidious because it can progress with minimal visible external indication, making it difficult to detect through routine visual inspection. This underscores the importance of implementing internal corrosion monitoring systems, such as ultrasonic thickness mapping or electrical resistance probes, for pipes operating in aggressive environments. The case also highlights the value of water chemistry management as a corrosion control strategy, which can be more cost-effective than material substitution in many applications.