Undulating oxide layer because of compressive growth stresses

The protection effectiveness of magnetite layers in relation to boiler corrosion

W.M.M. Huijbregts, A. Snel

5the International Congress on Metallic Corrosion, Tokio, 1972. (paper 08)

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Abstract

From failures in practical service and the results of exposure tests in small autoclaves as well as in an experimental 180 atu boiler, it has become very clear, that not only the chemical water treatment but also the strength of the oxide layer must have an important in-fluence on boiler corrosion.

Corrosion rates in hydroxide solutions were determined for chemically pure iron and several mild steel qualities. In the exposure tests a break-away in the oxidation curves was noticed. The pre-break-away parts of the oxidation curves could be described with a logarithmic oxidation equation, pointing to a mutual pore blocking mechanism and compressive growth stresses.

The tensile and compressive fracture strains of magnetite grown on mild steel appeared to be dependent on the hydroxide concentration, surface pre-paration of the samples and the kind of alkalizing chemical applied.

Defective tubes of operational boilers were examined very carefully. Three kinds of oxide scales were observed: very regular laminated, porous and blistered ones. The regular laminated scales can be ex-plained as mainly a result of spalling of the oxide during the many shut-downs of boilers. The porous and blistered scales are considered to develop particularly at the operating temperatures as a result of compressive growth stresses.

5. Conclusions

Though compressive growth stresses in magnetite have not yet been measured, the next phenomena very obviously point to the existence of these stresses:

1. The oxidation rates of iron and steel in 0,01 n NaOH and LiOH can be described with a logarithmic equation, based on a mutual pore blocking mechanism.
2. Formation of loose oxide in weakly alkaline solutions. This oxide breaks off from the samples during the exposure time.
3. Formation of pores and blisters in the oxide on tubes of operational boilers in case of heavy corrosion as well as on iron exposed in 0,5 n NaOH at 360oC in an autoclave.
4. Undulating of coarse crystalline magnetite.

Fracture of magnetite can easily occur during shut-downs on account of the thermal stresses caused by differences in thermal expansion of magnetite and steel. Very regular laminated magnetite is a result of frequently taking the boiler out of service.

From our studies it seemed that the mechanical properties of magnetite will play an important role in boiler corrosion, because of deposition of the loose broken oxide on places of high heat fluxes. On account of the deposits and the proceeding corrosion the tube wall temperature will increase, which results in higher oxidation rates and higher compressive stresses. So, once corrosion has started, the formation of thick porous or blistered oxide will become more dependant on the oxidation temperature and the mechanical properties of magnetite than on the kind of corrosive salts, although the formation rate can be influenced by the water quality as is the experience in boiler corrosion practice.