LESSONS FROM THE DEPTHS OF THE DEAD SEA
Can mild steel ever outlast stainless steel? It’s a question that challenges conventional engineering wisdom but under the right conditions, such as the ultra-saline, oxygen-starved waters of the Dead Sea, the answer might not be as clear-cut as it seems. This article examines two intriguing technical questions raised during the Sassda Fundamentals of Stainless Steel Course: first, whether Grade 316 stainless steel is appropriate for heating hard water; and second, whether mild steel might, in rare cases, offer greater durability than stainless steel?
The Importance of Grade Selection in Stainless Steel Applications
Stainless steel is not a single material but a family of more than 200 different grades, each developed for specific operating conditions and environments. Selecting the correct grade is fundamental to achieving reliable performance, optimal corrosion resistance, and long-term cost efficiency. This point was heavily emphasised during a recent session of the Sassda Fundamentals of Stainless Steel Course.
Two questions posed during the session highlighted just how critical grade selection can be. The first centred on selecting a suitable material for a vessel designed to heat and hold borehole water - typically high in mineral content - at temperatures above 50°C. The second posed a theoretical, but compelling challenge: is there any situation in which mild steel might outlast stainless steel?
Grade 316 and Its Application in Hard Water Heating
The first scenario described the need for a hot water cylinder operating at atmospheric pressure and intended to heat and store borehole water, which is known for its high hardness and elevated chloride levels. The initial material selection was Grade 316 austenitic stainless steel, a common and reliable choice for chloride-rich environments, thanks to its molybdenum content which enhances resistance to pitting corrosion.
From a corrosion protection standpoint, this selection is understandable. The molybdenum in Grade 316 provides improved defence against pitting, particularly when compared to Grade 304. However, corrosion protection involves more than just resistance to pitting. In this particular application, the presence of chlorides, elevated temperatures, and residual or applied stresses introduces another significant risk: Stress Corrosion Cracking (SCC).
The Hidden Threat of Stress Corrosion Cracking
SCC is an insidious form of corrosion that can cause sudden, catastrophic failure without significant warning. It occurs when three key factors are present simultaneously:
- A susceptible material - Austenitic stainless steels, especially those containing between 5% and 20% nickel (such as Grade 316), are inherently prone to SCC. Unnecessary costs, associated with removing, re-cutting, or treating affected products by taking proactive measures immediately after cutting.
- Tensile stress - This can be introduced through residual stresses in flat-rolled products, or through fabrication processes such as welding, cutting, and forming. In this case, additional stresses would be induced by the operational load of the heated water.
- A corrosive environment - Chloride ions are a common culprit, and their effect can be exacerbated in acidic water or at elevated temperatures.
Temperatures above 50°C significantly increase the risk of SCC in Grade 316. Given that the water in this application is expected to be chloride-rich, and the vessel is exposed to continuous tensile stress and heat, all three conditions for SCC are likely to be met.
While the pitting resistance of Grade 316 is suitable, its vulnerability to SCC makes it less than ideal for this specific use. Failure due to SCC can be unpredictable, irreversible, and costly, with consequences ranging from operational downtime to serious safety risks.
A Viable Alternative: Ferritic Grade 444
An alternative that warrants consideration is Grade 444, a ferritic stainless steel that shares several corrosion resistant features with Grade 316 but offers key advantages in this application.
Grade 444 contains approximately 18% chromium and 1.8 to 2.5% molybdenum, matching Grade 316 in general corrosion protection and pitting resistance. In fact, thanks to its chemical composition, Grade 444 offers a higher Pitting Resistance Equivalent Number (PReN) than Grade 316, providing superior performance in chloride-bearing environments.
Crucially, being a ferritic steel, Grade 444 is immune to SCC, eliminating one of the major risks posed by the use of Austenitics in high-chloride, high-temperature environments. In addition, it does not contain nickel, which not only reduces the likelihood of SCC but also lowers material costs, making it a more economical and stable option in price-sensitive applications.
From a fabrication standpoint, 2.5 mm material thickness is suitable for use with Ferritics and falls within the weldable gauge range, simplifying construction and assembly.
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.