[en] Due to the presence of H², H²O, CO and CH⁴ in impure helium environments surface reactions can occur causing oxidation, decarburisation or carburisation depending on the detailed level of impurities. The interplay between these different corrosion regimes can be represented on a Ternary Environmental Attack (TEA) diagram which shows that gas compositions causing decarburisation are separated from those causing carburisation by an area in which a protective Cr-based oxide is formed. This area gives protective oxides and represents the desirable composition range for HTR systems and it is argued that due to the presence of a hot graphite core, impurity levels in HTR systems will tend to fall in the protective corrosion regime at least up to a temperature of 900₀C. In any case, the TEA diagram provides a basis for developing methods for controlling impurity levels in an operating system to give acceptable corrosion behaviour. Experiments performed for periods of about 10000 h within the protective regime and in the lower band of gas composition specified as the reference PNP-Test-Helium environment show that the reference PNP alloys possess excellent corrosion resistance at 900₀C. (orig.)