Materials Transactions Online

Transactions of the Japan Institute of Metals, Vol. 12 No. 3 (1971) pp.184-191
© 1971 The Japan Institute of Metals

The Mechanism of Sulfur Dewpoint Corrosion*

Masamichi Kowaka** and Hiroo Nagano**

Electrochemical measurements were carried out to clarify the mechanism of sulfur dewpoint corrosion of steels, i.e., the corrosion potentials of various kinds of steels in the mixtures of sulfuric acid-commercial active carbon powder, and sulfuric acid-boiler deposit. The effect of commercial active carbon and unburned carbon in boilers on the corrosion was discussed from these results.
Furthermore, the anodic polarization curves for steels in sulfuric acid at high temperature and concentrations were measured so as to investigate the relation between the anodic polarization characteristics and the resistance for sulfur dewpoint corrosion.
The experimental results are summarized as follows:
(1) In the mixtures of 65∼85% H2SO4 and active carbon at 110°C, the corrosion potentials of the resistant steels containing chromium or boron shifted exceedingly to the noble potential direction, while those of non-corrosion resistant steels such as mild and copper steels hardly changed during the test period.
It means that the corrosion resistant steels are easily passivated, and non-resistant steels are kept active. The commercial active carbon has the same influence on the dewpoint corrosion as unburned carbon in boiler deposits.
(2) The primary passive current densities of the anodic polarization curves in 80 and 85% H2SO4 at 110°C decreased with increasing chromium content in steels. The low primary passive current densities were corroelated well with the high resistivity of the steels for sulfur dewpoint corrosion.
(3) The mechanism of sulfur dewpoint corrosion of steels seems to be composed of the corrosion process of three steps. The first step is the corrosion of steels in sulfuric acid at relatively low temperatures and concentrations, the second step is in sulfuric acid at high temperatures and concentrations, and the third step is in boiler deposits containing both sufficient unburned carbon and sulfuric acid at the same temperatures and concentrations as in the second step.
The corrosion resistant steels containing chromium or boron are passivated at the third step, and their corrosion rates become extremely low.

(Received December 16, 1970)

* This paper was originally published in Japanese in J. Japan Inst. Metals, 34 (1970) 32.

** Central Research Laboratories, Sumitomo Metal Industries, Ltd., Amagasaki, Japan.

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