technical insight
Optimising Performance: Surface Finish and Post-Weld Treatment In corrosive applications, surface finish plays a critical role in material performance. For Grade 444, a 2B mill finish is recommended as it offers the smoothest surface, which helps to reduce crevice sites and biofilm formation, thereby enhancing corrosion resistance. Equally important is post-weld treatment, particularly in areas affected by fabrication heat. Proper passivation and surface restoration are essential to restore corrosion resistance and maintain material integrity in service. Mild Steel vs. Stainless Steel: A Rare Exception? The second question raised during the session took a different direction, challenging a long-standing assumption: Is there a situation where mild steel could actually outlast stainless steel? On the surface this seems implausible, but under specific environmental conditions, it may be possible, particularly in oxygen-depleted environments where the mechanisms of corrosion for both materials are disrupted. Understanding the Corrosion Mechanisms Mild steel corrodes by oxidation in the presence of oxygen, forming rust (iron oxide). Conversely, stainless steel relies on the presence of oxygen to maintain its passive chromium oxide layer, which protects the underlying metal from corrosion. If the oxygen content in the environment is extremely low, both processes slow dramatically. In fact, without oxygen, mild steel has no catalyst for rusting, and the passive layer on stainless steel can degrade without being replenished. A fascinating example of this can be found in the Dead Sea, one of the most saline and dense bodies of water on Earth. With salinity levels as high as 34.2% and significantly reduced oxygen concentrations, conditions are such that
carbon steel exhibits minimal corrosion when fully submerged. A 1990 study on corrosion and scaling in the Dead Sea basin revealed that steel pipework in submerged applications experienced unexpectedly low corrosion rates. This is further supported by anecdotal evidence: mild steel ladders, rails, and fixtures submerged in the Dead Sea often show surprisingly little rust, defying expectations for such a saline environment. Does Mild Steel Actually Last Longer? While these findings are intriguing, they do not conclusively prove that mild steel will outlast stainless steel. The real takeaway is that both materials experience minimal corrosion due to the lack of dissolved oxygen. However, stainless steel’s passive layer is still vulnerable in low-oxygen settings, and once breached, corrosion will proceed similarly to mild steel. Thus, it is more accurate to say that in unique submerged environments like the Dead Sea, mild steel may last just as long as stainless steel - not longer. Such scenarios are extremely rare and not representative of most industrial or domestic applications. Context specific selection These two technical discussions underscore the importance of context-specific material selection. Grade 316 is widely used and trusted, but it is not a one-size-fits-all solution, particularly in environments conducive to SCC. Ferritic Grade 444, while less well-known, offers excellent corrosion resistance and immunity to SCC, making it a strong candidate for hard water heating systems. As for mild steel outperforming stainless steel, such cases are rare and dependent on extreme environmental conditions like those found in the Dead Sea. Nonetheless, they remind us that materials engineering is never absolute - it must always be grounded in the realities of the application.
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Issue 2 – 2025
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