The use of corrosion inhibitors in formulas for boiler cleaning and for cooling water is well established but an increasing number of regulations and the abolishment of various well-tried corrosion inhibiting actives put up a new challenge. Chromate, molybdate, nitrite, phosphonate and other inhibitors are being abandoned for either ecotox or cost reasons. In this presentation, the potential of organic corrosion inhibitors for industrial water treatment is demonstrated. After a brief survey on structure and function of organic corrosion inhibitors, the function and the mechanisms of corrosion protection are discussed for acetylene derivatives in the light of their application in boiler cleaning and scale removal. Immersion tests and electrochemical measurements are used to monitor the effect of the inhibitors and their synergy with other additives.
Several well established corrosion inhibitors are currently being discussed concerning their potential to harm the environment (Table 1). These corrosion inhibitors are likely to be prohibited in the future for specific applications. Among these applications, boiler cleaning and scale removal play a dominant role. It is the aim of this presentation to explain what alternatives might work. It is not expected that new compounds based on transition metals, phosphorous, and borine derivatives will be accepted as replacements for inhibitors that must be abandoned. Consequently, in this presentation the use of organic corrosion inhibitors is discussed.
(available in full paper)
Boiler cleaning and scale removal operations are frequently carried out with acids that are combined with corrosion inhibitors. Table 2 summarizes corrosion inhibitors considered in this work. In Figure 1, recent work on the feasibility of various corrosion inhibitors for the use in acids is summarized. 2
The anticorrosive effect of various organic corrosion inhibitors is tested in an immersion test. Steel coupons (G 10150 3, 10 cm x 2 cm) are immersed into hydrochloric acid (pH 2) at 80°C for 6 hours. The anticorrosive effect is determined as corrosion inhibition efficiency CI
CI=¿m0-¿mi/¿m0
where ¿mi is the mass loss of the coupon after immersion in the corrosive medium with corrosion inhibitor and where ¿m0 is the mass loss of a reference after immersion in the corrosive medium without corrosion inhibitor. The results of the corrosion test are summarized in Figure 1. Acetylene derivatives like PP and DEP clearly reveal a much stronger anticorrosive effect as compared to propargyl alcohol, PA, a standard pickling inhibitor for treatment of steel in acids. The cationic polymer CP reveals an anticorrosive effect stronger than PA but not as strong as PP and DEP. Anionic species like the phosphonate HEDP and the polyamino acid P(ASPA) do not exhibit an anticorrosive effect.
Apparently, some of the most powerful corrosion inhibitors for acidic media are based on acetylene derivatives. For acetylene derivatives, an anodic corrosion inhibition mechanism is discussed. Mechanistically, an Fe-catalyzed polymerization reaction of the acetylene derivative is assumed that yields a polymer layer adhering firmly on the metal surface to be protected. Another group of particularly powerful corrosion inhibitors are cationic polymers.