316 stainless steel is a great material for standard fittings and valves. It is strong, fairly corrosion resistant, and plentiful in the fittings market. However, due to its austenitic nature, it is prone to a condition known as Stress Corrosion Cracking (SCC). SCC is caused when a piece of material is under tensile stress in a corrosive environment. Though this is a greater issue at warmer temperatures, given the right conditions, SCC can occur even as low as 77°F (25°C) as in the case of most swimming pool applications. SCC is not the only threat to austenitic stainless products. They are also relatively susceptible to pitting in medium to highly corrosive environments such as use at sea. For these applications, a higher level alloy such as 2205 Duplex Stainless or Incoloy® alloy 825 could be a better solution.
Duplex stainless steel is called Duplex because it combines both austenitic (ex. 304 and 316 Stainless) and ferritic (ex. 430 Stainless) phases of stainless steel together in a 1 to 1 ratio. This melding of the two phases causes the steel to inherit beneficial characteristics of both.
Duplex stainless is much stronger than the austenitic forms of stainless steel. In fact, its minimum yield stress value is about double that of the austenitic varieties. Therefore, thinner material can be used, opening the possibility of weight savings overall. 2205 Duplex can, with due care, be welded with relative ease. It also maintains its strength at low temperatures reaching as far as -58°F (-50°C). Finally, Duplex has much higher resistance to corrosion and SCC. Where 316 stainless has a Pitting Resistance Equivalent Number (PREN) of 24, 2205 Duplex sports a PREN of 35, meaning it can effectively be used in salty ocean spray and other mid to high corrosion environments.Though Duplex is a very versatile material, when compared to other highly corrosion resistant materials, it also remains fairly inexpensive. This is because it contains a smaller proportion of nickel than most other materials with equivalently high PREN values.
Duplex does have a few potential problems that you should be aware of before committing to it as your material of choice. To address the lesser issue first, -58°F is its lowest minimum working temperature. However, if the temperature drops much lower than that, the material can become brittle making Duplex unsuitable for cryogenic work.The larger issues come in the form of phases that can develop in the steel and cause brittleness in the material.
Incoloy® alloy 825 is nickel based super alloy. It is largely comprised of Nickel, Chromium, and Iron, but also contains smaller amounts of Molybdenum, Copper, Titanium, Carbon, Manganese, Sulfur Silicon, and Aluminum. It is well suited to handle low temperature, even cryogenic environments; and it can withstand up to 1,000°F (540°C) before problematic phase formation is a concern.Beyond just temperature, Incoloy® 825 is even more corrosion resistant than Duplex. With a PREN value of 40.8, alloy 825 is highly resistant, not only to oxidation but also chemical reduction. This makes it sufficient for use, not just in swimming pools and off shore work, but even fully submerged in salt water or environments containing sulfuric and phosphoric acids.
Alloy 825 looks like stellar option, and it is. However, Incoloy® tends to be much higher cost than Duplex and other stainless steel alloys because of its high nickel content. If top notch corrosion resistance is the crux of you operation, you’ll want to be sure and budget well for any nickel super alloy.On top of the cost, Incoloy® does not have the same incredible tensile strength as Duplex material, making it slightly less versatile. Though, its strength can be improved somewhat by use of certain cold working techniques in its manufacturing process.
If you are looking for a more corrosion resistant material to use in an off shore or other mildly corrosive environments, Duplex 2205 is a cost efficient solution to SCC and pitting. However, if you are working with high heat, cryogenic temperature, or corrosive gasses and acids, be prepared to spend a bit more for those nickel alloys such as Incoloy® alloy 825.
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