Mechanical and Corrosion Testing in Welding Inspection

16 November 2017

Multiple challenges greet a welding inspector when a project calls for his services. Have the right welding procedures been applied? Is the joint geometrically spot-on? These questions and more are checked off until the construct is certified as structurally rock-solid. Wait a minute, though, what about the mechanical and corrosion testing? It seems like the works not done just yet, not until we identify these twin threats.

Identifying and Addressing Corrosive Damage 

Rust needs water and air to begin its spread. That’s a problem because these two corrosion enablers are literally everywhere. Worse yet, if this is an oil rig or some coastal framework, then there’s salt in the water. Salt accelerates material corrosion. So how do we inspect a weld area for this nasty oxide spread? First of all, iron corrosion appears as a highly visible stain. The orange oxide blooms and spreads. Left to eat into the weld faces, the rust will cause structural failure. Weld decay takes place, especially when the joint is pitted with tiny crevices. Stress fractures and ‘knifeline’ flaws also indicate a serious underlying condition. Address the issue by employing a corrosion inhibitor. Additionally, remove the corrosion source.

Mechanically Assured Weld Inspections 

Mechanical integrity is a longstanding issue here, one that requires exhaustive attention. If the joint isn’t robust, as required by the engineering specs, then it becomes the weakest link in a critically important chain. Weak links crack, then they pull down the rest of that chain until it comes crashing down. In this case, a certified welding inspection visually evaluates the welding technique and the physical characteristics of the weld. Are there stress-induced micro-fractures on the surface of the joint? Is the solidified connection distorted due to torsional stress? Is it beginning to shear? The mechanical properties are further examined by calling in a subsurface testing rig, so all under-the-skin flaws are uncovered and recorded.

The act of applying a weld can actually amplify corrosion sensitivity on this occasion. The heat changes the metal surface ever so slightly, and the formerly heat treated alloy now becomes susceptible to rust. Then there are the environmental conditions, plus the installation circumstances to evaluate. If dissimilar metals are touching, galvanic corrosion occurs. Likewise, poor gas shielding practices can result in an oxidation gradient, so this flaw will exhibit a signature pitting pattern. Mechanically speaking, the corrosion weakens structural integrity. Eliminate the environmental factor, if possible. Otherwise, call in the welder, get him some extra training, and eliminate all poor jointing practices, including that poor gas shielding issue.

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