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Corrosion Susceptibility of Welded Joints

May 27, 2015

Welded joints have an increased susceptibility to corrosion due to microstructure changes to the metal that occur during the welding process.

One of the many advantages of using composite pipe supports is that they do not require welding for installation. In addition to being time consuming and requiring highly trained personnel, the welding process has many variables that complicate the process such as filler metals, weldment design, welding sequence, and many others. Another major cause of concern is the increased susceptibility to corrosion that welded joints can have compared to their un-welded counterparts.

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This corrosion susceptibility can largely be attributed to microstructure changes that the metal experiences during welding. A weldment includes the base metal, the heat affected zone (HAZ), and the fusion zone, with many more microstructurally distinct regions in between.

As a joint is welded, the metal is exposed to different temperatures for different lengths of time and this variation introduces thermal gradients into the welded metal. Each region of the weld experiences different extents of melting and varied solidification velocities, which is why several diverse microstructures arise. The differences in microstructure include, but are not limited to, crystallization variances and the precipitation of secondary phases.

Due to these diverse microstructures, the corrosion resistance of welds is often inferior to that of un-welded metal. The different microstructures can be at different stress levels. High stress regions are at a higher energy state and can become anodic to the more stable areas on the weld or base metal, which creates a corrosion cell. This type of corrosion is commonly referred to as preferential weld corrosion.

Both autogenous and heterogenous welds are susceptible to corrosion, although autogenous welds generally exhibit an increased corrosion resistance in comparison. Hetergenous welds, welds made with a filler metal that is different than the base metal, are at a higher risk for corrosion because an electrochemical potential could be induced by the difference in metals and then galvanic corrosion could occur.

The welding process has many degrees of freedom and thus proper execution is critical in order to avoid preferential weld corrosion. Welding is an important process that is integral to many applications, but it is not necessary for the installation of your pipe supports. Potential corrosion issues could be averted by using adhesive, instead of welded joints. Not only do adhesive joints avoid problems due to weld corrosion, but they also are less time consuming and do not require highly trained personnel like welding does. APP composite pipe supports use APP Epoxy as opposed to welding for installation.

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