Internal Tube Corrosion:
Internal corrosion is predominantly influenced by the chemical composition of the process fluid, process and metal temperatures, fluid velocity and tube metallurgy. Some critical species include sulfur compounds and organic acids (naphthenic acid). The level of these species in the fluid influences the type and rate of corrosion on the internal surface of the heater tubes. Sulfur compounds in particular promote sulfidic corrosion, which can manifest itself as localized and general wall thinning. Similarly, corrosion from organic acid can appear localized in turbulent regions or general thinning in areas. The corrosion rate can be aggravated by other fluid components like chlorides, the type of sulfur compounds, and the presence of hydrogen. Sulfur compounds, in the presence of hydrogen, usually increase the corrosivity of the process.
Fluid and metal temperatures influence the corrosion rate. The highest tube metal temperature predominantly occurs at the fire side front face of the radiant tube where the heat flux is greatest. The corrosion rate profile often follows the heat flux profile. Differences in the corrosion rate along the length or around a cross section of a tube are often the result of temperature differences between locations. An example of temperature influence is the increase in corrosion rate with a rise in temperature to 750°F (399°C) for processes with sulfur compounds. Above 750°F (399°C) the corrosion rate decreases due to stable sulfide scale that inhibits further corrosion. Figure 10 is a convection tube that failed from internal corrosion attributed to high-temperature sulfidic attack.
High-velocity fluids, fluids containing particulate, or tubes with two-phase flow can increase the corrosion rate by stripping away protective scale and exposing fresh metal to continue the corrosion process. Corrosion from organic acids and sulfur compounds are significantly influenced by fluid velocity. Particular attention should be given to high turbulent regions.
Tube failures resulting from corrosion are generally due to local stress rupture in which the wall thickness becomes too thin and is overstressed at the metal operating temperature. These failures can appear as small leaks through pits or as “fish-mouth” ruptures if the thinning is general or if it is longitudinal grooving.
External Tube Corrosion:
External corrosion of the tube depends on the heater atmosphere and temperatures. Generally, the external surface of the tube will corrode from oxidation. The heater atmosphere contains excess oxygen necessary for combustion of the fuel at the burners. Oxidation rates for a metal increase with increased temperature. Refer to API Std 530 for additional information on oxidation. Oxidation may either be a localized condition or extend over the entire length of the tube inside the heater. Excessive oxidation and scaling is usually the result of operating the tubes above recommended levels. This could be the result of over-firing the heater or internal fouling of the tubes, which increase the tube wall temperature. Combustion deposits may have the appearance of oxide scale, but they can be distinguished by checking them with a magnet. Oxide scales are magnetic, whereas combustion deposits are not.
Other types of corrosion attack are possible. Heaters operating with insufficient oxygen or fuel-rich can cause corrosion from the resulting reducing environment. Depending on the type and quality of the fuel, corrosion could occur from sulfidation or carburization. Acid attack can result from the combustion of heater fuels depending upon the sulfur content of the fuel. When the gas or fuel oil has a high sulfur content, one of the combustion products formed and deposited on the outside surfaces of the tubes is a sulfate. This sulfate is harmless during operational periods, but when the deposit is allowed to cool it becomes highly hygroscopic and absorbs moisture from the air, hydrolyzing to produce sulfuric acid, which corrodes the underlying metal.
When the fuel has a high vanadium content, metal at temperatures above a critical point in the range from 1200°F (649°C) to 1400°F (760°C) is subject to very rapid attack from low-melting vanadium based compounds (vanadates) and sodium-vanadium compounds (sodium vanadates). The vanadates and sodium vanadates deposit on the hot metal surface, melt and act as a fluxing agent to remove the protective oxide scale on the tubes The cycle repeats itself as oxide and deposit builds back up on the tube.
Convection sections where flue gas dew point temperatures occur during operations suffer metal loss because of acid material from the products of combustion. Metal loss on the exterior of convection tubes may be difficult to evaluate because of inaccessibility.