CoatingsPro Magazine

MAR 2016

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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Page 63 of 84

COATINGSPRO MARCH 2016 63 Figure 2. A scanning electron micrograph of the specimen shown in Figure 1 is presented after rinsing the section with methanol. A nodular texture, typical of electroless nickel, is shown. a scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS). T he process of analyzing a failed coating starts w ith the collec- tion of background information. It is important to know the speci- fied requirements for the part and the coating. T hat information is often available in the form of a part draw ing and/or referenced material specifications. It is also important to know what the expected perfor- mance of the part is and what the actual performance was compared to the expectation of the coating. A ny know n or inadvertent changes made to the manufacturing process should be reported to the failure analyst. Examples include vendor, design, material, and processing changes. T he next step in the failure analy- sis process is the visual examination of the part. Features to be noted, recorded, and photographed in the visual examination include the presence of blisters, f laking, damage to the part, the presence of residues, discoloration, corrosion products, and corrosion pits. It is critical for the analyst to carefully document, record, and identif y the pieces of the failed coating and the damage present on the failed part in its as-received condition. Since the failed coating w ill be handled, excised, and examined in the laborator y, it can be critical to know the condition of the sample, as-received, because the sample can be damaged and sectioned during the investigative process. Electron microscopy is a critical step in the metallurgical analysis of failed parts. Te electron microscope is the best means of examining the surface texture of the part. Visual and low-power light microscopy can comple- ment the inspection as the coloration of residues and contaminants are not evident in the SEM. Te imagery from the SEM is almost exclusively in black and white. Metallography is an import- ant step in the metallurgical failure analysis process as well. Examination of the metallographic specimen can help verify whether proper adhesion was present between the coating and the substrate. Metallography can also identify whether contaminants were present between the coating and substrate that could have had a delete- rious efect on the part's performance. Case History In the case of a water heating unit, the coating on the tubes was prematurely failing and the nickel plating on the units would frequently fake of. A magnifed view of a tube, by use of a low-power binocular light microscope, is shown in Figure 1. A gray coloration is evident in the view. Inspection via SEM illustrates the nodular texture of the plating, common for electroless nickel, as shown in Figure 2. Te heating units had experienced leaks in the feld due to corrosion-related failures. Te piece shown in Figure 1 was plated in copper to help protect the nickel coating during the sample prepa- ration process. W hen the specimens are polished, rounding can occur, making the measurement of the nickel plating thickness less accurate. Similar features were present along a braze joint in the section. Te presence of a holiday in the nickel plating along a dark feature embedded in the braze material was noted. Similar holidays were present through the nickel plating near contaminants along the bend radius of the sample wall. T he SEM-EDS is a useful tool in determining the relative elemen- tal composition of contaminants. T he results of the EDS analyses are presented in Table 1. Te mounting epoxy was frst analyzed via EDS, as the materials are known to smear during the sample preparation process. Te EDS detected primarily carbon with a lesser amount of oxygen along with trace amounts of iron and chlorine. Te base metal (substrate), copper layer, and nickel layer (topcoat) were also analyzed via EDS. In the base metal, the EDS detected primarily iron with smaller amounts of chromium and nickel along with small to trace amounts of manganese, molybdenum, and silicon. Copper from the plating Flaked Nickel Plating

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