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Direct Determination of Carbon on Metal Surfaces

“Direct Determination of Carbon on Metal Surfaces” AFP/SME Technical Paper FC78-584. Dearborn, MI: Association for Finishing Processes/Society of Manufacturing Engineers. 1978.

King, Arthur E.

ABSTRACT

A method for the direct determination of carbon on steel and other metal surfaces has been developed. Organic materials and some forms of elemental carbon on the surfaces are oxidized to carbon dioxide (CO2). After removal of potentially interfering substances the CO2 is determined using a CO2 coulometer. It was found that at temperatures below 600˚C carbon contained in steel is not oxidized. The accuracy of the method is ±0.02mg C/ft2 (0.0002g C/m2). Recoveries of carbon by the new method were higher than with indirect techniques. The direct method is faster and easier to use than the indirect methods.

INTRODUCTION

During the manufacture and handling of sheet metal the surfaces may become contaminated by a variety of materials. These contaminants may interfere with the adhesion of surface coatings and reduce corrosion resistance. It is desirable to test all lots of material to determine the extent of surface contamination.

Hospadaruk (1) has shown a correlation between the surface carbon contamination and corrosion resistance. The carbon may be from surface oils or amorphous carbon or graphitic carbon deposited by pyrolysis during coil annealing. A procedure in current use (1) to determine surface carbon on steel is to swab samples with as glass fiber filter paper saturated with 50% hydrochloric acid (HCl). The HCl is driven off and the carbon residue on the swab determined. Surface contamination has also been determined by washing samples with chloroform, evaporation of the chloroform and weighing the residue. Both the chloroform wash and the HCl swab methods suffer from long analysis times, potential loss of volatile components and incomplete removal of surface contaminants.

This paper describes a new method for determination of surface carbon using direct oxidation of surface contaminants to CO2. The CO2 formed is determined by coloumetric titration. Interference from the carbon within steel does not occur. The method is rapid, simple to perform and does not risk loss of volatile components or incomplete removal of contaminants.

EXPERIMENTAL

Chloroform Wash/Weigh Method

Samples 12in. (300mm) x 12in. (300mm) were washed with chloroform and the chloroform and all particles removed collected. The chloroform was evaporated and the residue weighed. The weight of the residue was determined to within 0.5mg.

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