High temperature oxidation is not a real common type of failure in our industry, but it can and does happen when temperatures exceed design maximums. All metals oxidize, even at room temperature, and in many cases that slow oxidation process actually protects the metal from rapid oxidation. Even rusting is a low temperature oxidation process. But at higher temperatures, oxidation can proceed fast enough to produce excessive scaling and thereby inhibit the usefulness of steels and alloys at elevated temperatures. For carbon steel the oxidation temperature limit is usually in the vicinity of 900F (482C) - 950F (510C) range and above the 1000F (538C) -1050F (565C) range carbon steel starts to become limited in usefulness as a construction material because of excessive scaling over time. However, even before we reach these temperatures, carbon and low alloy steels are often limited by other high temperature metallurgical concerns such as creep rate, potential for creep-cracking, short-term tensile overload, and some forms of embrittlement like graphitization. Temperature cycling and intermittent service exposure may also affect the material selection. So oxidation is usually NOT the most limiting aspect of high temperature materials selection.
For higher temperature oxidation resistance, as well as strength, we begin to alloy our steels with chromium and molybdenum to increase their usefulness in high temperature applications such as furnace tubes, furnace outlet piping, hot hydroprocess equipment and catalytic reaction equipment. Steels such as 1.25Cr-0.5Mo, 5Cr-0.5Mo and 9Cr-1.0Mo are three of the most useful and widely available low alloy steels for hot services, but there are other specialty low alloy steels. Above the temperatures where even these steels will scale excessively, we have to keep increasing the Cr content and start adding Ni in order to stabilize the oxide layer for cyclic services. Some companies make good use of12&17Crsteelsupintothe1400F(760C)-1500F(815 C) range. For even higher temperature services, there are a multitude of high temperature alloys, with the austenitic stainless steels being the most widely used. API RP 571 has an excellent chart providing estimated oxidation rates for the most commonly used high temperature steels in our industry divided into twelve 50 degree temperature ranges between 900 and 1500 F.
Infrared thermography and surface thermocouples are two widely used methods of monitoring for temperatures that may exceed design conditions of construction materials. When high temperature oxidation failures do occur, it’s usually because the excessive scaling is not obvious or “hot spots” are not obvious. They may be on furnace tubes that are not within sight of a furnace view port or may be occurring underneath external insulation. Refractory linings can fail, causing steel pressure retaining construction materials to be exposed to excessive temperatures. If the OD is insulated, these excessive temperatures may not be apparent.
Do you understand and have you assessed your risks of high temperature failure with furnace components or other equipment operating at high temperatures, especially those components that may be inadvertently subjected to higher than design temperatures? Catastrophic Oxidation (Fuel Ash Corrosion) Catastrophic oxidation can occur when certain contaminants are present in a high temperature environment, ie inside furnace fireboxes in our industry. Those contaminants are typically vanadium pentoxide with sulfur oxide or sodium sulfate. When these contaminants are present in the combustion atmosphere, liquid slags can form on components operating above 1000F (538C) which can cause exceedingly high rates of corrosion (sometimes up to 1000 mpy). If you have this problem, you probably already know it because of the high rate of corrosion and the glassy, hard slag on the surface of the components being inspected. Furnace tubes may not be affected if they operate below the molten slag temperature, but support structures and tube hangers are often exposed to fuel ash corrosion. There are a variety of methods to reduce the risk of catastrophic oxidation; several of which are covered in API RP 571.
Would your management of change (MOC) process catch any changes in fuel oil firing that might introduce vanadium contamination into your fireboxes? Does your RBI assessment take into consideration the potential for fuel ash corrosion?