Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/792600
55 NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS PERFORMANCE MARCH 2017 range of 6.7 to 7.3, which makes the com- pact Fe 3 O 4 layer difficult to form. In addi- tion, CO 2 can react with the steel pipe and destroy the protective oxide layer in this pH range. It is difficult to characterize the inter- nal surface product of this heating system. Visual analysis shows that the precipitation of the rust water is an orange-yellow color, and this color relates to the composition in the rust phases. XRD characterization of rust water precipitation in Figure 3 shows that the rust water precipitation is mainly c o m p o s e d o f i r o n o x i d e s a - Fe O O H , γ-FeOOH, and Fe 3 O 4 . Usually, a-FeOOH is a dark brown color and γ-FeOOH is yellow- orange in color. 10 While γ-FeOOH is active, Fe 3 O 4 is typically compact and dense. More- over, γ-FeOOH can be reduced to Fe 3 O 4 as the corrosion process proceeds. 10 The re- duction of γ-FeOOH in the cathodic pro- cess can promote the corrosion of steel. Therefore, the precipitate color indicates that a compact corrosion product layer has not yet been formed on the internal sur- faces of the steel water pipe. It is reported that a compact rust layer can be formed when the heating system has operated for ~20 months. 5 Polarization Curve Characterization The polarization cur ves of unrusted Q235 steel in high-salt and low-salt water were viewed (Figure 4) to further under- stand the corrosion behavior of the pipe steel. The two curves exhibit almost the same shape, indicating similar corrosion behavior of the steel in the two types of water. The cathodic reduction process is controlled by the DO limiting diffusion, and the anodic process is controlled by the steel dissolution. For the anodic process of the two curves, the steel shows higher corro- sion current in the high-salt water than in the low-salt water, indicating higher corro- siveness of the high-salt water. Further- more, the corrosion potential of the steel in the high-salt water is more electronegative than in the low-salt water, which indicates greater corrosiveness. Therefore, it can be concluded that th e high-salt make-up water is responsible for the equipment cor- rosion. At the same time, the presence of Cl – and SO 4 2– ions play a key role in the cor- rosion process, and they can preferentially FIGURE 4 Polarization curve characterization of noncorroded Q235 steel in high-salt and low-salt water. adsorb at the specific sites on the internal pipe surfaces due to shorter ionic radii and high adsorption energy. 9 Corrosion Mechanism The corrosion of the newly installed family heating system is the combined re- sult of the delivery water chemistry and the steel components. 11 The DO is responsible for the cathodic reaction of the steel struc- ture, which produces hydroxide (OH – ) ions. The ferrous ions (Fe 2+ ) from the anodic reac- tion can unite with OH – ions from the ca- thodic reaction. The corrosion process can be illustrated by Equations (1) and (2): 12 Fe → Fe 2+ + 2e (1) (Anodic dissolution reaction) H 2 O + 1/2O 2 + 2e → 2OH – (2) (Cathodic reduction reaction) The dissolved Fe 2+ ions can be hydro- lyzed with OH – into a deposit of iron (II) hydroxide [Fe(OH) 2 ] on steel pipe internal surfaces, as shown in Equation (3): Fe 2+ + 2OH – → Fe(OH) 2 (3) Fe(OH) 2 can be further oxidized into iron (III) oxide-hydroxide [Fe(OH) 3 ] depos- its, shown in Equation (4): Fe(OH) 2 + 1/2O 2 + H 2 O → Fe(OH) 3 (4) Fe(OH) 3 is nonadherent in nature and reddish-brown in color. CO 2 can promote the corrosion of steel as the initial steel corrosion product, and Fe(OH) 2 , can react with CO 2 , which contrib- utes to the formation of soluble bicarbon- ate (HCO 3 – ), shown in Equation (5): CO 2 + H 2 O → H 2 CO 3 + H + + HCO 3 – (5) With the presence of O 2 , HCO 3 – will be oxidized to Fe(OH) 3 , which frees the CO 2 as shown in Equation (6): HCO 3 – + Fe(OH) 2 → Fe(OH) 3 + CO 2 (6) The regenerated CO 2 can again react with Fe(OH) 2 , and this cyclic process enables the continuous progress of corrosion. 13 When the water contains SO 4 2– , it can lower the water pH value and accelerate the corrosion process, as seen in Equation (7): 14 Fe 2+ + SO 4 2– → FeSO 4 (7) This cyclic process can produce sulfuric acid (H 2 SO 4 ), and thus accelerate the corro- sion process, shown in Equation (8): 4FeSO 4 + O 2 + 6H 2 O → 4 γ FeOOH + 4H 2 SO 4 (8) Corrosion Analysis of a Newly Installed Family Heating System