CorrosionDOE-HDBK-1015/1-93GENERAL CORROSIONRev. 0CH-02Page 17RemovingCorrosiveAgentsChemistry control in the form of removal of corrosive agents from a system is a widelyused method. One method is using deaerators to remove dissolved oxygen and to alesser extent carbon dioxide. Treating the water by softening and demineralizationremoves the dissolved solids and reduces the conductivity.ChemicalAdditionChemical additions to a system that alter the chemical reaction or tie up a particularcorrodant is a common method of control. Filming amines (organic compounds that arederivatives of ammonia) accomplish protection by forming adhering organic films onmetal surfaces to prevent contact between corrosive species in the condensate and themetal surface. Phosphates and sodium hydroxide are used to adjust the system pH andremove hardness. CorrosionofAluminumThe corrosion of aluminum is dependent upon a vast number of variables. These variablesinclude environment, temperature, alloy in question, flow velocities, impurities present in theenvironment, and chemistry conditions to which it is exposed. An additional factor that affectscorrosion is pretreatment.Many of the factors are controlled by design and construction, such as alloy type, temperature,and flow velocities. Pretreatment, soluble and solid impurities, and chemistry are within thecontrol of the operator and will be discussed in this text.Experiments have shown that prefilming limits corrosion on aluminum-clad fuel assemblies. Inthe tests conducted, the ratios of oxide film thickness for nonprefilmed and prefilmed elementswere on the order of 2 to 3 and in some cases even greater.Impurities are major contributors to the corrosion of aluminum. In most cases studied, the majorsource of contaminants has been makeup water systems. Corrosion products from other plantmaterials also contribute to the overall inventory of ionic and solid impurities. Finally, organicimpurities from the resin used in ion exchangers have been detected. These occur in someinstallations because of the type of resin used and the particle filters normally installed on the ionexchanger effluents. This problem has been reduced by improved resins and installing filterscapable of removing smaller particles.Chemistry controls, including pH, dissolved oxygen, and conductivity, greatly influence theformation and propagation of the oxide film on aluminum surfaces. Dissolved oxygen iscontrolled for essentially the same reasons as for the corrosion of iron. Conductivity is aquantitative indication of the impurities present in the system, and pH theoretically dictates thevalue of conductivity.
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
