APPLICATION NOTE Failure analysis of corroded steel surface Root cause assessment using the Axia ChemiSEM Pickled Hot Rolled Steel Coil Hot Rolling Black Hot Rolled Steel Coil Pickling Line Figure 1. Hot-rolled steel sheet production process. Introduction Hot rolling processes (Figure 1) produce an oxide layer (also known as scale) on the surface of the steel strip. This scale must be removed before further processing is performed. Shot-blasting may be used to remove scale from discreet parts or for some sheet processing such as stainless steel. An acid bath dissolution process, called pickling, is the most common method for oxide scale removal on carbon steels. The pickling consists of the immersion of the hot-rolled steel sheet in a highly concentrated, temperature-controlled acid solution in order to remove surface oxides (visible in Figure 2, left). Acids commonly used for pickling include hydrochloric, sulfuric, nitric, and hydrofluoric, among others. A combination of acids may be used to optimize the material cleaning while preserving the base metal of a wide range of steel alloys. The etched steel strip is then rinsed in water multiple times to ensure no pickling residues are left on the surface and then dried in warm air. The sample characterized in this application note is a structural steel that underwent a pickling process and has been left in the etched condition (no oil, phosphate, or primer coating). However, stains have been found after the drying step, requiring further investigations to assess the cause. Figure 2. Hot-rolled coils or “black bands” in the left image show black oxide scale formed during reheating and red rust which formed at ambient temperature. The right image shows a hot-rolled steel coil after pickling

Methods The root cause assessment was achieved using the Thermo Scientific™ Axia™ ChemiSEM, a newly designed scanning electron microscope (SEM) that provides live quantitative elemental mapping through its always-on energy dispersive X-ray spectroscopy (EDS). This means the operator can view all the needed elemental information at any time without switching techniques. Discussion Figure 3 shows a picture of the defective surface. Scanning electron microscopy combined with the chemical information provided by EDS is needed to investigate the defect to assess the presence of morphological abnormalities...

The ChemiSEM image in Figure 6 shows the distribution of all the present elements. The presence of iron is expected, while the presence of oxygen highlights where the sample is oxidized. The presence of chlorine, however, is unexpected and needs further investigation. For an instant confirmation of the chlorine distribution, the Axia ChemiSEM allows you to hide certain elements in order to show only the element of interest (Cl in this case). Quick area analyses were then executed on another foreign particle and on a clean area of the etched steel (shown in Figure 8) to quantify the contaminants...

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