Korrosionsverhalten von feuerverzinktem Stahl in erwärmten Wasser1

Corrosion behaviour of hot galvanized steel in warm water Pipe specimens made of commercial grade galvanized steel and zinc have been tested in comparison to pipe specimens having modified zinc coatings. The specimens were corroded in 11 test lines with warm water at approx. 60 °C and cold water with continuous and intermittent flow. Dortmund town water with polyphosphate and orthophosphate additions in individual test lines was used for the tests. One test line was operated with warm water from a water heater with cathodic vessel protection according to the Guldager method. The investigation included visual assessment, determination of mass loss, and electrochemical measurements. The main results were as follows: 1. The localised corrosion tendency is not determined by the amount of potential ennoblement, but rather by the inhibition of the cathode reaction on the surface layer formed, which can be read off the cathodic current density vs. potential curve. The inhibition of the cathode reaction can be affected both by variations of the material and by variation of the water quality. 2. The tested materials with modified coatings featured throughout a poorer corrosion behaviour in warm water than commercial zinc coatings according to DIN 2444. 3. The phosphates used for the investigation induce an inhibition of the cathode reaction in zinc. Zinc-iron alloy phases in warm water fail to ensure an appropriate effect. A treatment of the water according to the Guldager process ensures a strong inhibition of the cathode reaction both with zinc and especially with zinc-iron alloy phases and induces an important improvement of the corrosion behaviour of galvanized steel in warm water.