16 TECHNICAL PAPER
Figure 3 is a cross sectional electron microscope image of a chemically attacked metal anchor. Even though there may be different layers similar to growth rings in a tree, essentially there are three definite zones; the original metal, a crust consisting of the product of corroded metal, and the refractory. This is shown in the simple schematic of Figure 3. By analysing these layers a better understanding of the actual operating conditions under which the anchor has been exposed to can be gained. It also has to be realised that the product of metal corrosion (crust) has a much greater volume than the starting material, and this creates tremendous stresses within the refractory lining, resulting in cracking (See Figure 2). Once voids are created within the refractory lining gas can penetrate deeper into the lining, aggravating the process of destruction.
In terms of the different forms in which chlorine can be present in the unit, gaseous HCl is less corrosive than alkali chlorides condensed on the steel. Where both gas and salt deposits exist, corrosion accelerates. Higher oxygen partial
pressure also increases the severity of
rate. Furthermore, in the presence of condensed alkali chlorides, such as NaCl or KCl, additional reaction of these metal oxides will take place to produce even higher concentrations of chlorine gas.
2NaCl + Fe2 4NaCl + Cr2
+ ½O2 = Na2 +2 ½O2 Fe2 = 2Na2 O4 + Cl2(g) CrO4 + 2Cl2(g)
Addition of sulphur, often found in the fuel, has an accelerated effect on chlorine attack by increasing the chlorine partial pressure close to the metal. This type of high temperature corrosion is known as accelerated oxidation or sulfidation.
Sulphidation occurs as SO2 in deposits. Chlorine is
corrosion, and, therefore, reducing conditions are less severe than oxidising conditions. The reaction mechanisms for both reducing and oxidising conditions are explained schematically in Figures 4 and 5.
Firstly, chlorine will attack the metal to form FeCl2 . In its gaseous form,
the iron chloride diffuses away from the steel, oxidises (in oxidizing atmosphere) and releases the chlorine again for further reaction with the steel. If no oxygen is available some iron will diffuse into the surrounding atmosphere as iron chloride, while the remaining iron will exist as iron chloride scale.
The increase in corrosion is attributed to the following mechanisms; i) lowering of first melting temperature and associated ease of reaction
in the gas reacts with of alkali chlorides found released by sulfation of chloride-containing
deposits according to the reaction: 2(K,Na)Cl + SO2
+ O2 = (K,Na)2 SO4 + Cl2
From the above reactions it is clear that even small chlorine concentrations in the gas environment can have a much larger effect on the corrosion rate, depending on the actual conditions.
ii) increased chlorine partial pressure trapped under the deposit (crust)
Base on the above; higher temperatures would favour the corrosion reaction, even though it can occur as low as 400°C and maybe even lower. Oxygen in the atmosphere will react with the formed iron chlorides and release chlorine gas, thereby, increasing the partial pressure of chlorine close to the metal and dramatically increasing the corrosion
Figure 3: Schematic showing the main zones of a chemical attacked anchor with micrograph of a stainless steel specimen exposed to potassium and chlorine
It is evident that from the attack mechanism described above that corrosion is possible well below the normal operating temperature of stainless steels used as anchorage and that few steels would actually survive this attack. The nickel super alloys are most probably the nearest solution, but may not be economically justified. Choosing from the range of available options; 310 stainless steel is most likely the more cost effective choice as it has 24-26%Cr and 19-22%Ni. The addition of molybdenum to the steel as is found in 316 (16-18%Cr; 10-14%Ni, 2-3%Mo), could also provide some corrosion resistance against chlorine.
However, for all these materials continuous use in 425-860°C range are not recommended due to carbide precipitation, a phenomena called “sensitization” as discussed in Part 2 of this series. This is when chromium in the base metal joins with carbon to form chromium carbides at the grain boundaries, which reduces the chromium available in the matrix to help the steel produce the protective chromium oxide film.
Figure 6 shows the dramatic effect of chlorine corrosion on a sensitized
NOVEMBER 2014 ISSUE
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