Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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68 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS SELECTION & DESIGN N A 24-in (610-mm) water injection line failed by cracking. Testing indicated that the pipe material did not contrib- ute to failure. Deterioration of the ex- ternal coating led to severe localized corrosion attack and a gradual thin- ning of the pipe wall. Cracks were ini- tiated from pitting on the outer sur- face. Low cycle fatigue stresses were created from improper suspension and fxation. The failure was caused by corrosion fatigue. N e a r l y a l l e n g i n e e r i n g s t r u c t u r e s experience some form of alternating stress and are exposed to harmful environments during their service life. Corrosion fatigue is caused by the combined action of cyclic stress and a corrosive environment. 1-3 The fatigue process is thought to cause rupture of the protective passive film, leading to corrosion. Corrosion continuously reduces the fatigue strength of the material and, if a cyclical load is present, a crack will start even under very light loads. This leads to a long incubation period before a crack starts and then a slow-growing fracture, where the crack propagation per stress cycle is almost infinitesimal. 4-5 The excessive stresses may be caused by restraint and load changes during shut- down or restart of water pumping to the line. Trace contaminants, such as sulfur in an atmosphere, can greatly alter the rate of attack. Corrosion fatigue can be reduced by alloy additions, inhibition, and cathodic protection , all of which reduce pitting. Because corrosion fatigue cracks initiate at the metal surface, surface treatments like plating, cladding, nitriding, and shot peening have been found to improve the materials' resistance to this phenomenon. 6-8 Experimental Work A 24-in (610-mm) water injection line failed by cracking. The failed pipe segment was examined by visual inspection, elemen- tal analysis for material , microstructure investigation for both normal and failed structure, and the seam weld line. Also, cracks were evaluated by stereoscopic exam- i n ati on , s c a n ni n g el e c tron mi cro s c o py (SEM), and energy dispersive spectrometry (EDS) analysis. Mechanical properties of the pipe material were measured and compared to the standard values. Results and Discussion Visual Inspection Figure 1 shows an overview of the failed pipe segment. The crack was located parallel to the pipeline axis at 90 degrees from the lon- gitudinal double seam weld line. Although the weld line has lost its coating layer and is cov- ered by an oxide scale layer, it is still in good condition with no connection to the failure. The rupture origin is located at a mid- point of the crack; severe thinning around the maximum opening zone of the crack was observed. The indication of general corro- sion star t ed from th e origin p oint and extended on both sides in the form of metal loss, scales, and rust. The crack origin site indicated a reduction in thickness of up to 76%. Cracks were originated from the mini- mum reduced thickness, propagated by iner- tia at the normal designed operating pres- sure, and stopped when stress relief was attained. Corrosion Fatigue Failure of a Water Injection Line in Oil Operation MohaMed hanaF y el-Sayed, CMRDI, Cairo, Egypt

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