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54 MARCH 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 3 oxygen (O 2 ) in the cathodic process, which parallels the steel anodic dissolution reac- tion during the corrosion process. It is re- ported that the oxygen concentration must not exceed 100 µg/L in the make-up water or 20 µg/dm 3 in the delivery water. 5 Water pH Value The water pH value has a great inf lu- ence on the corrosion of heating systems, as the pH value represents the hydrogen ion (H + ) concentration in the water solu- tion. 6 Lower pH values mean higher H + con- centrations in the solution , w hich can cause severe corrosion of steel pipe. Gener- ally, lowering the pH value of the solution increases the corrosion rate of steel pipe. Carbon Dioxide Available data show that in a heat sup- ply system, the CO 2 -induced corrosion ef- fect is ~20 times more aggressive than O 2 - induced corrosion. 7 Moreover, for most areas of northern China, 8 the DO concen- tration of make-up water is ~1.37 mmol/L and C O 2 c on c entration i s ~4 mm o l/L (nearly four times the O 2 content). In addi- tion, the CO 2 -induced corrosion is much more complicated than O 2 corrosion. Corrosive Ions Th e corrosiven ess of th e wat er in- creases when certain anions are present in the electrolyte. The presence of Cl – and SO 4 2– ions in the water make the water more corrosive. Cl – and SO 4 2– ions play a key role in the corrosion process by forming ferric chloride (FeCl 3 ) or ferrous sulfate (FeSO 4 ). These can preferentially adsorb at the specific sites on internal pipe surfaces due to smaller ionic radii and high adsorp- tion energy. 9 Rust Water Precipitation Analysis If a protective oxide corrosion product layer forms on the steel pipe internal sur- face, it can resist the progression of further corrosion and reduce the steel pipe corro- sion rate. For corrosion product on internal surfaces of the steel water pipe, a compact layer of iron (II,III) oxide (Fe 3 O 4 ) is benefi- cial for resisting further corrosion. How- ever, the pH value of the make-up water for the heat supply system is usually in the FIGURE 2 Corrosion at the heating end typical of each room. FIGURE 3 XRD patterns of the precipitate in the rust water. MATERIALS SELECTION & DESIGN