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Corrosion Mapping of Steel Reinforced in Concrete Exposed to a Galvanizing Process Atmosphere

J.M. Malo and J. Uruchurtu
Instituto de Investigaciones Eléctricas
Reforma 113, Col. Palmira
Temixco, Morelos 62590 Mexico
email: "Jose M. Malo"

Abstract

Corrosion of steel reinforced concrete was investigated for beams and columns part of an industrial building where a hot dipping galvanizing process takes place. Inspection using a reference electrode allowed to produce half-cell potential maps. Analysis of half-cell potentials indicate two sources of corrosion in beam structures, the outdoor environment and fumes produced in the galvanizing process indoors. Identification of structures showing major damage was made which allowed to establish a rehabilitation and repair program.

Introduction

Hot dip galvanizing is a process in which an adherent, protective coating of zinc is developed on the surfaces of iron and steel products by immersing them in a bath of molten zinc [1,2]. Hot dip galvanized coatings are produced on a variety of steel mill products using fully mechanized, mass production methods. Continuous galvanizing consists of surface preparation, fluxing and galvanizing. At the surface preparation stage scale and rust are generally removed by pickling in hot sulfuric acid. The final cleaning of the steel before actual galvanizing is performed by a flux that dissolves any oxide film left on the steel after pickling and facilitates the wetting of the steel by the molten zinc. The parts are dipped through a flux blanket that float on the molten zinc. The flux blanket is made of ammonium chloride along with foaming agents and fume suppressing compounds. After the parts are thoroughly cleaned and fluxed, they are immersed in the molten-zinc bath, which is normally maintained at a temperature of 835 to 855 oC. After galvanizing parts are cooled with air and allowed to dry before stacking or storage.

Therefore, the galvanizing process involves the handling of various chemical compounds some of which can evolve fumes, and depending on the effectiveness of the extraction unit working on the baths, can reach areas beyond the process. For the case considered, galvanizing process fumes over several years of production had reached adjoining concrete structural members, due to an inconsistent use of the extraction unit, sometimes left out of work for long periods.

The industrial building structure considered is located in a marine zone, about 500 m. from the seashore, and served the purpose of containing a continuous hot dipping galvanizing process of steel sheets and storage of raw and processed goods. The over 40 years old structure consisted of an array of 15 m (49 ft) long beams standing on 7 m (23 ft) height columns (Figure 1 and 2).

Figure 1. Overview of columns and beams and relative location of galvanizing process.

Some of the members displayed corrosion degradation in isolated sections in the form of concrete cracks, removed sections of concrete and rebar exposure. A particular concern was the visible bending of a couple of beams indicating a loss of structural integrity and a reinforced column to which metallic straps had been added to provide additional support. However, most of the members, although near the process, had no apparent corrosion. Therefore half-cell potential readings was suggested as a mean to extend the understanding of corrosion degradation on the building structures following a non-destructive approach. Although columns and beams close to the pickling and galvanizing baths, were expected to have the higher degree of degradation, there was the need to indicate the extent of corrosion damage and therefore identify the structures for which rehabilitation was essential for safety of the personnel laboring at the plant.

Figure 2 shows a general view of the building under inspection, some sections of the ceiling showed evidence of concrete removal and particularly for the beam at the center, cracks had developed at the joint with the column and displayed a light bending at its middle section.

Figure 2. General view of building's columns and beams.

Inspection Procedure

Inspection procedure was based on ASTM's standard method [3] for half cell potentials of reinforced steel in concrete. This method allows to determine the corrosion activity of the reinforcing steel.

Measuring elements

Half cell reference. A copper-copper sulfate reference electrode was used. Temperature reading were made at approximately 30 oC and a digital voltmeter with a 10 MW input impedance was used for potential measurements. An electrical junction solution of Na2SO4 was used attaining a low electric resistance between the external concrete surface and the half cell.

Measuring Procedure

Spacing between measurements. A 30 cm spacing and 20 cm height grid was drawn on the concrete beams and columns to indicate points of potential measurements. This spacing allowed at three potential readings on each side face of the beam and one on the lower face. Electric contact to rebar was attained using a compression clamp, by either connecting directly to exposed rebar or removing the concrete in the most affected section. ASTM's ranges [3] for potential interpretation were further divided to better asses corrosion activity.

Results

Although several approaches to corrosion assessment of reinforced concrete structures have been proposed [4-6], such as corrosion rates and resistivity measurements or chemical analysis, it was decided that simple half-cell potential measurements should tell apart members in good structural condition from those in need of rehabilitation or replacement, as the corrosion problem demanded.

Figure 3 depicts typical half-potentials readings for beam 2AB, which was located close to the galvanizing stage of the process. Length in meters is included on top of the beam drawing. Three potential readings were made every half meter on both side faces of each beam plus one on the bottom face. Therefore, for each beam about 210 potential values were collected. For the case considered, a wide range of potential values was obtained for the beam due to its relative position to the galvanizing and pickling bath. Particularly, the right section of the beam which is closer to the process, displays predominance of active potentials, under -400 mV vs. CSE. In contrast, at the opposite side of the beam, relatively high potentials, above -200 mV vs. CSE, were recorded. The middle section of the beam therefore, corresponds to a transition section where the influence of the fumes decreased with distance from the process.

It is interesting to notice the active potential values taken place at the top of the beam, often regardless of the distance from the process. Therefore a different source of corrosion was proposed to explain this behavior. Since the building is located in a marine zone with a high humidity level, corrosion attack can also be expected from natural sources, particularly stimulated by chloride ions transported from the nearby sea. In fact readings taken above the beam, standing on the building roof, confirmed the existence of active potentials in most of the beams upper face.

Figure 3. Half-cell potentials measurements for beam 2AB.

Figure 4 shows contours of equal potentials values that result from interpolating values of beam in Figure 3. Here a graphical delineation of areas of different corrosion activity allows a rapid identification of probable damaged and safe areas. As explained above, due to the location of the beam, which extends away from the galvanizing bath, this structure displays the whole range of corrosion activity, depending on the distance away from the galvanizing process.

Figure 4. Equipotential contour map for a beam in Figure 3.

Figure 5 shows a plan view which includes several of the beams and columns in the vicinity of the galvanizing displaying equipotential contour maps. Beams and columns with corrosion activity in the bottom face of beams are typical of member structures affected by the galvanizing process. Fumes from the pickling bath can act as a chemical neutralizers decreasing the high pH of the structural members and therefore affecting the stability of the concrete. Flux chemicals can also contribute to degradation of members by convection of chloride ions from the galvanizing bath to the concrete external surface, subsequently diffusing inside the concrete and eventually affecting protective oxides at the rebar/concrete interface.

Beams negligibly affected by the process fumes showed a different corrosion activity pattern, displaying corrosion in isolated areas in the upper section of beams. Here corrosion activity is related to the outdoor environment, due to the permeability of the concrete, the low integrity of the external coating applied to the roof, and the high humidity and chloride contribution from the environment.

Figure 5. Plan view of beams and columns surrounding galvanizing process.

The results obtained and their interpretation helped to classify structural members into beams and columns with major damage and in need to be rebuilt, namely beams 2AB and 2BC and column 2, and those which could be rehabilitated, namely 3AB and ABC, and those with minor damage. Results also helped to emphasize the importance of insulating the access from aggressive agents from the environment to the structural members and the need to avoid concrete contact with galvanizing process fumes by proper use of the extraction unit.

Repairs proceeded according to corrosion activity areas detected, rebuilding members with major damaged, repairing by removing loose concrete and introducing a light plastic roof supported by metallic frame in space between beams.

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