Method for treatment of iron-based metal surface exposed to superheated steam

Abstract

The present invention provides a surface treatment method for suppressing the formation and growth of superheated steam oxide scale on an iron-based metal surface exposed to superheated steam, comprising treating said iron-based metal surface with a surface-treatment agent which comprises a polyoxy saturated aliphatic mono- or di-carboxylic acid or a salt thereof and an amine compound represented by the following formula (I):Z(CH2CH2NH)nCH2CH2NH2  (I)wherein Z represents H or OH or NH2 group, and n is an integer of 0-5.

Description

TECHNICAL FIELD[0001]The present invention relates to a method for treatment of an iron-based metal surface exposed to superheated steam. More particularly, the invention relates to a surface treatment method for suppressing the formation and growth of steam oxide scale on an iron-based metal surface exposed to superheated steam.BACKGROUND ART[0002]The inner surfaces of steam pipes, such as superheater pipes, main steam pipes, reheater pipes, reheat steam pipes of thermal power plants and the like, are exposed to superheated steam, which is generated by superheating with superheaters or reheaters, saturated steam produced in boilers. Therefore, the inner surfaces are oxidized and coated with steam oxidation scale during a long period of operation. Since the steam pipe base materials (iron-based metals) are different in thermal expansion coefficient from steam oxide scale, thermal stress occurs between them by change in temperature at the start and the stop of the boilers, thereby ex...

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Benefits of technology

[0012]By the treatment method according to the invention, the formation and growth of steam oxide scale is suppressed on an iron-based metal surface exposed to superheated steam. As the result, it is possible to reduce the frequencies of irregular stop and chemical cleaning of a plant (a power plant, for example) due to steam oxide scale exfoliation and therefore achieve the improvement in the reliability and operation efficiency and the reduction of the maintenance costs of a plant equipped with pipes carrying superheated steam. It is also possible to reduce the burdens on the environment.

Problems solved by technology

The exfoliated steam oxide scale can be deposited at U-bent or other portions of the steam pipes to occlude them or can strike against turbine blades to damage them, thereby stopping the power plants irregularly and therefore reducing the generation efficiency or increasing the maintenance and repair costs.

Thus, the formation and growth of steam oxide scale on the inner surfaces of steam or other pipes can cause problems relating to plant reliability or maintenance.

Formation and growth of steam oxide scale, which could be considered a corrosion phenomenon by high temperature steam, cannot be suppressed by conventional anticorrosion using a deposition coating, a neutralizing amine such as morpholine, cyclohexylamine or a film-forming amine.

However, for performing chemical cleaning, a large-scale work can be required in which only a steam piping system to be cleaned is once cut off from the other piping systems (which are adversely susceptible to cleaning agents) and after cleaning, they are welded to restore.

In the case of cleaning the inner surface of a large-scale piping system used in a power plant for example, chemical cleaning requires a large amount of chemical cleaning solution and a large amount of the resultant drained cleaning solution must be cleaned up.

Thus, chemical cleaning has problems of very high costs and heavy burdens on the environment.

Boiler steel pipes having good steam oxidation resistance (e.g., Patent Literature 1) and a method for suppressing oxidation of a steam pipe inner surface by setting the electrical potential within a certain range at the inner surface of the pipe (Patent Literature 2) are under consideration, but are not yet widely applied due to high cost.

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