Formwork Rust

Steel formwork is used in prefabrication operations because of its robustness and reusability, as well as its ability to produce prefabricated units with smooth surfaces. On these steel molds, brownish-reddish rust spots sometimes form while the concrete hardens. This rust could lead to brown or red spotty discolorations on the concrete surface lower the concrete element's appearance, Fig. 1 .

Source: CONCRETE PRODUCER MAGAZINE
Publication date: May 1, 2007

By Peter Boos and Reiner Haerdtl

Introduction

Steel formwork is used in prefabrication operations because of its robustness and reusability, as well as its ability to produce prefabricated units with smooth surfaces. On these steel molds, brownish-reddish rust spots sometimes form while the concrete hardens. This rust could lead to brown or red spotty discolorations on the concrete surface lower the concrete element's appearance, Fig. 1 . Removing these discolorations by sanding and resurfacing is costly and time-consuming.

Reasons for corrosion

There are several reasons for metal corrosion, and thus different forms: contact corrosion, crack corrosion, inter-crystalline corrosion, pitting corrosion, etc. The corrosion of iron and/or steel is an electrochemical process in the presence of water and oxygen. Metal corrosion occurs at the spot with the higher electro-negative potential. Here, the metal ions dissolve from the surface into the solution and when they collide with hydroxide ions they precipitate as iron hydroxide.

The resulting iron minerals are formed, depending on temperature and air humidity. Due to constant recrystallization, no permanent protective rust layer is formed on the surface that would prevent further corrosion.

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