The main processes of metallic corrosion in primary circuits of advanced high-temperature gas-cooled reactors (HTGRs) at temperatures above 800 °C are oxidation, carburization, and decarburization. These are caused by helium impurity traces of H2O (causing oxidation and decarburization), CO (causing oxidation and carburization), and CH4 (causing carburization). At the very low partial pressures of these impurities, the three processes happen independently, leading to a multitude of corrosion effects, which can be classified in terms of active and passive regimes. In an active regime internal corrosion proceeds rapidly— usually linear with exposure time—thereby severely affecting the structural integrity of the alloy. Passive regimes are characterized by stable oxide layers, which either completely inhibit internal corrosion or limit it to a parabolic dependence with exposure time. These passive and active regimes can be related to absolute partial pressures and partial pressure ratios of the main gaseous impurities, H2O, CO, and CH4. This relationship is illustrated in the form of ternary corrosion maps termed Ternary Environmental Attack diagrams. For each temperature and alloy, such a diagram can be constructed from existing results and used for outlining the likely shape of the passive area for the given temperature. A set of diagrams defines a common passive area for a given alloy over a temperature range, which can be compared with the range of gas compositions expected in the HTGR primary circuit. If it is found that the area representing the expected primary circuit environment is not fully enclosed in the passive corrosion area for commercially available candidate alloys, it will either be necessary to control the primary circuit impurity concentration to such levels that the gas composition is completely shifted into the passive corrosion area, or it will be necessary to develop new alloys with passive corrosion areas big enough to engulf any given primary circuit environment.