BEHAVIOR IN HOT WATER AND STEAM

26.3 BEHAVIOR IN HOT WATER AND STEAM

The good resistance of zirconium to deaerated hot water and steam is of special

ZIRCONIUM

in general for prolonged periods without pronounced attack at temperatures below about 425 ° C (800 ° F). The rate of attack is characteristically low at fi rst; but after a certain time of exposure, ranging from minutes to years depending on temperature, the rate suddenly increases. This “ transition ” phenomenon is reported to occur for pure or impure zirconium after a weight gain on the order

of 3.5 – 5.0 g/m 2 , and similar additional accelerated oxidation may occur at still higher weight gains [19] . It occurs at lower temperatures if the zirconium is con- taminated with nitrogen ( > 0.005%) or with carbon ( > 0.04%) [20] . The damaging effect of nitrogen in this respect is offset by alloying additions of 1.5 – 2.5% tin in combination with lesser amounts of iron, nickel, and chromium.

Marker experiments indicate that oxidation proceeds by diffusion of oxygen ions toward the metal – oxide interface (anion defect lattice). It has been sug- gested on this basis that trivalent nitrogen ions in the ZrO 2 lattice increase anion - defect concentration, thereby increasing the diffusion rate of oxygen ions. But were this the mechanism, oxidation in oxygen would also be affected, which is not the case. Adding to the complexity is the observation that alloyed tin appre- ciably shortens corrosion life of zirconium in water; but tin in the presence of small amounts of alloyed iron, nickel, or, to a lesser extent, chromium again increases corrosion resistance, with the combination overcoming the detrimental effect of alloyed nitrogen.

The mechanism accounting for the transition phenomenon is not well under- stood. It has been explained on the basis of cracks forming in the oxide because of stresses accumulating as the oxide thickens. However, an increased corrosion rate does not occur when the metal oxidizes in oxygen except for much longer times and much thicker oxide fi lms. Hydrogen formed by the decomposition of

H 2 O during reaction appears to exert a dominant role, especially that portion which dissolves in the metal, causing higher oxidation rates [19] . X - ray data for the oxides that form in H 2 O show a monoclinic modifi cation of ZrO 2 either before or after the breakaway time, but with some evidence that the initial oxide is a tetragonal modifi cation [20] .

The oxidation behavior of Zircaloy - 2 in water and steam is shown in Fig. 26.2 . In brief, zirconium is resistant to the following:

1. Alkalies, all concentrations up to boiling point, including fused caustic.

2. Hydrochloric acid, all concentration up to boiling point. Embrittlement of metal and higher corrosion rates occur above boiling temperatures under pressure.

3. Nitric acid, all concentrations up to boiling point, including red fuming acid (S.C.C. may occur under slow strain rate conditions [14] ).

4. Sulfuric acid, < 70%, boiling.

5. Phosphoric acid, < 55% H 3 PO 4 , boiling.

6. Boiling formic, acetic, lactic, or citric acids.

REFERENCES

Figure 26.2. Corrosion of Zircaloy - 2 in high - temperature water and steam, showing transi- tion points [19] .

Zirconium is not resistant to the following:

1. Oxidizing metal chlorides (e.g., FeCl 3 and CuCl 2 ).

2. Hydrofl uoric acid and fl uosilicic acid.

3. Wet chlorine.

4. Oxygen, nitrogen, and hydrogen at elevated temperatures.

5. Aqua regia.

6. Trichloroacetic or oxalic acids, boiling.

7. Boiling CaCl 2 , > 55% [21] .

8. Carbon tetrachloride, 200 ° C (explodes) [22] .