Corrosion Resistance Properties of INCOLOY Alloy 800
High nickel-content alloys do, in general, offer excellent resistance to multiple forms of corrosion. However, in more hostile environments – particularly at high temperatures – some can struggle to maintain that same exceptional performance. The Nickel Institute estimates that a temperature rise of just 10 degrees can increase the corrosion rate up to 12x, demonstrating just how important it is to select an alloy that exhibits the right properties for the intended application.
Oxidation, carburisation and sulphidation can all occur in high-temperature environments, increasing the risk of surface pitting, stress corrosion cracking and sulphide diffusion. At a time when there’s a huge amount of pressure to operate with greater efficiency, process temperatures are getting higher and higher, sparking a need across many different industries to identify an affordable solution that maintains strong corrosion resistance even at high temperatures.
Fortunately, there is such a solution: INCOLOY alloy 800, which has succeeded in setting the industry standard for strong, versatile nickel alloys with unrivalled corrosion resistance – even at high heat.
What is INCOLOY alloy 800?
INCOLOY 800 is a form of INCOLOY alloy developed by the Special Metals Corporation. Combining nickel, chromium and iron – along with trace amounts of other metals – it’s renowned for its strength, durability and resistance to corrosion from oxidation, carburisation and sulphidation.
Tobe categorised as INCOLOY 800, an alloy must be made up of:
- 30-35% nickel
- 19-23% chromium
- 39.5%+ iron
- 0.15-0.6% aluminium
- 0.15-06.% titanium
- Up to 0.10 carbon
INCOLOY alloy 800 was first developed in the 1950s. Today, it’s not only used as an alloy in its own right, but as the foundation of a number of other alloys in the INCOLOY 800 range. Some of these alloys offer enhanced corrosion resistance, which we’ll explore in more detail later in this article.
A high-performance alloy
INCOLOY alloy 800 has earned a reputation for being a high-performance super-alloy, capable of resisting corrosion in some of the most hostile environments. And it all comes down to its composition. With the high nickel content resulting in good resistance to sulphidation, the iron reducing the risk of internal oxidation and the chromium demonstrating excellent resistance to both oxidation and general corrosion, the critical chemical balance in INCOLOY alloy 800 makes it superior in resistance to many types of corrosion. This includes scaling and stress-corrosion cracking, particularly chloride stress corrosion due to the formation of metal chlorides.
In many cases, the corrosion rate of INCOLOY alloy 800 has been shown to begin rapidly, then ease to a slower, steadier rate. This is due to the fact that polarisation resistance increases with prolonged exposure. Ultimately, this provides INCOLOY alloy 800 with a much slower corrosion rate than stainless steel 304, making it a better option for certain applications.
Studies have found that INCOLOY alloy 800 exhibits high corrosion resistance when exposed to molten carbonates, molten eutectic salt deposits, hydrogen peroxide and hydrogen chloride in moist air. And while research in this area may be limited, it’s also been found to have high resistance against sulphuric acid, corroding at a rate of just 0.51mm/y (20mpy) when exposed to 5% sulphuric acid at a temperature of 50 degrees, according to research undertaken by the Nickel Institute.
Applications
With INCOLOY alloy 800 having such pronounced corrosion resistance properties, especially at higher temperatures, it is most commonly used in applications where high heats are common, such as industrial incinerators, furnace tubes, heat processing equipment and electrical heating elements. It can even be used for steam generator tubing and is commonly found within paper processing mills.
Overall, INCOLOY alloy 800 is an excellent choice for environments up to 816 degrees. Where high levels of corrosion resistance are required at even greater temperatures, other alloys within the INCOLOY 800 series may be utilised as an alternative, such as 800H, 800HT, or even INCOLOY 825.
Enhanced resistance
As briefly mentioned earlier, INCOLOY alloy 800 is the basis of a number of other nickel alloys. The closest of which is 800H. In fact, INCOLOY 800 and INCOLOY 800H are so similar that they were once both categorised as INCOLOY alloy 800, with one labelled ‘Grade 1’ and the other ‘Grade 2’. Grade 2 contained slightly higher levels of carbon than Grade 1, soon becoming known as INCOLOY 800H.
INCOLOY 800HT is simply a variation of this higher carbon alloy, with a greater quantity of both aluminium and titanium. With these additions, corrosion resistance at even higher temperatures – well above 816 degrees – is possible. The rate of corrosion for INCOLOY alloy 800H and 800HT in a refinery furnace is 0.15mm/y or 6.0mpy. To put this in perspective, the corrosion rate in the same environment for stainless steel 310 is 0.23mm/y or 8.9mpy. For stainless steel 309, the rate is 2.15mm/y or 84.5mpy. And stainless steel 304 experienced complete oxidation in the environment.
INCOLOY alloy 825 is, perhaps, the most different from INCOLOY 800. With the addition of molybdenum and copper, this is a higher-density alloy with greater levels of corrosion resistance. General corrosion resistance is improved due to the higher nickel content, and resistance against sulphuric and phosphoric acids is greater due to the presence of molybdenum. The addition of copper enhances corrosion resistance in room-temperature environments, making INCOLOY alloy 825 a popular choice in situations where corrosion resistance is required in varying temperatures.
Improve corrosion resistance with INCOLOY 800
INCOLOY alloy 800 piping and tubing is readily available. However, a growing number of suppliers are shifting to INCOLOY 800H and INCOLOY 800HT simply for the fact that the composition is practically identical – meaning there’s no loss in strength or durability – but the slight amendments to the alloy spark even greater corrosion resistance, especially in hostile high-temperature environments.
Wondering if INCOLOY alloy 800 is right for you? Get in touch with us for expert support and guidance.
