2205 Duplex Stainless Steel vs. Incoloy825 Alloy
This article discusses how 2205 Duplex Stainless Steel and Incoloy825 Alloy compare to 304 and 316 stainless steel in mildly corrosive environments.
When should you consider replacing 316 stainless steel valves and fittings with a specialty alloy?
316 stainless steel is an excellent material for standard fittings and valves. It is strong, corrosion resistant, and in plentiful supply in the aftermarket. However, due to its austenitic nature, it is susceptible to a condition called stress corrosion cracking (SCC). SCC occurs when a material is subjected to tensile stresses in a corrosive environment. While it is more of a problem at higher temperatures, under the right conditions, SCC can occur even at temperatures as low as 77°F (25°C), as is the case in most swimming pool applications. SCC is not the only threat to austenitic stainless steel products. They are also relatively susceptible to pitting corrosion in moderately to highly corrosive environments, such as in offshore use. For these applications, a higher grade alloy such as 2205 Duplex or Incoloy Alloy 825 may be a better solution.
2205 Duplex Stainless Steel
Duplex stainless steel is so called because it combines austenite (such as 304 and 316 stainless steel) and ferrite (such as 430 stainless steel) phases in a 1:1 ratio. The fusion of these two phases gives the steel the beneficial properties of both.
Duplex stainless steel is much stronger than austenitic stainless steel. In fact, its minimum yield stress value is about twice that of austenitic stainless steel. As a result, thinner material can be used, potentially saving weight overall. Duplex 2205 can be welded with relative ease if care is taken. It also maintains its strength down to -58°F (-50°C). Finally, duplex stainless steel has a higher resistance to corrosion and stress corrosion cracking. 316 stainless steel has a Pitting Resistance Equivalent Number (PREN) of 24, while 2205 duplex stainless steel has a PREN of 35, which means it can be used effectively in salty ocean spray and other moderate to high corrosion environments. While duplex stainless steel is a very versatile material, it is also quite affordable compared to other highly corrosion resistant materials. This is because it contains a smaller proportion of nickel than most other materials with comparably high PREN values.
Is there anything Duplex can't do?
Before choosing Duplex stainless steel as your material of choice, you should be aware that it does have some potential issues. To address the smaller issues first, -58°F is its minimum operating temperature. However, if the temperature drops much lower than this, the material becomes brittle, making Duplex stainless steel unsuitable for cryogenic service. The larger issues manifest themselves in the phases that can form in the steel, causing the material to become brittle.
The first is called Sigma Phase. This property is caused by improper cooling of the steel during the manufacturing process. The higher the alloy content of a duplex steel material, the more care must be taken to avoid the formation of Sigma Phase, which is a greater problem for the higher grade alloys in the family of duplex steels known as Super and Ultra Duplex Steels. The best way to avoid Sigma Phase is to always source your duplex steel alloys from a reputable manufacturer to ensure that proper cooling methods are used.
Another disadvantage of duplex steel is Alpha Prime Phase, also known as 475° embrittlement. This phase is most commonly found at 475°C and forms when the steel is exposed to extreme heat. While the phase gets its name from 475°C, it has historically been found to form at temperatures as low as 300°C or 572°F. In short, duplex steel should not be used in extreme heat environments above 572°F.
Incoloy Alloy 825
Incoloy 825 alloy is a nickel-based superalloy. It is primarily composed of nickel, chromium, and iron, but also contains small amounts of molybdenum, copper, titanium, carbon, manganese, sulfur silicon, and aluminum. It is well suited to handling low and even cryogenic environments; it can withstand temperatures up to 1,000°F (540°C) before problematic phases form. In addition to its high temperature resistance, Incoloy 825 is even more corrosion resistant than Duplex. With a PREN value of 40.8, the 825 alloy is not only highly resistant to oxidation, but also to chemical reduction. This makes it suitable not only for use in swimming pools and offshore operations, but even for full immersion in salt water or environments containing sulfuric and phosphoric acids.
Disadvantages of Incoloy 825
The 825 alloy may seem like a good choice, and it is. However, due to its high nickel content, Incoloy tends to cost much more than Duplex and other stainless steel alloys. If top-notch corrosion resistance is key to your operation, you will want to make sure you budget for any nickel superalloy. In addition to cost, Incoloy does not have the same amazing tensile strength as Duplex materials, so its uses are slightly reduced. However, its strength can be slightly improved by using certain cold working techniques during the manufacturing process.
Summary: If you are looking for a more corrosion resistant material for offshore or other mildly corrosive environments, Duplex 2205 is a cost-effective SCC and pitting solution. However, if you work in high or low temperatures or in corrosive gas and acid environments, be prepared to spend more for a nickel alloy such as Incoloy Alloy 825.
