Adsorption of Copper on Stainless Steel

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Abstract
SUMMARY Under some conditions, dissolved copper becomes associated with stainless steel in a form that is readily available for contamination of milk. A small amount of copper-nickel alloy, either used or cleaned with stainless steel, can cause disproportionately large copper contamination of milk. Adsorption of dissolved copper on stainless steel was appreciable in the pH range of 6 to 10, especially in the presence of hypochlorite. The amount of copper that adsorbed increased with the concentration of copper in solution, the tempera- ture, and the time of exposure. Copper adsorption was slight or absent at low or Very high pH or in the presence of a chelating agent (EDTA). Adsorbed copper was not removed by thorough rinsing with water, but largely was removed by a chelating agent {EDTA) or milk, or exhaustively by hot nitric acid. In both laboratory and commercial experiments, copper contamination has been observed in milk processed in equipment made entirely of stainless steel. A source of contamination was found to be copper-bearing alloys in other equip- ment cleaned in the same circulation cleaning circuit as the stainless steel. Part of the copper that dissolved from these alloys during cleaning became associated with stainless steel surfaces in a state readily removed by milk. This association of copper with stainless steel is referred to herein as adsorption, although its nature has not been established. Swartling (10) described conditions in some butter factories wherein copper from the hot-water supply became deposited on equipment and later contami- nated butter. Raw water took up considerable amounts of copper from heaters with copper pipes. Then, when the butter-manufactu ring equipment was cleaned with hot water, copper deposited on the surface of the equipment, and later dissolved in cream and butter. ttensley, Long, and Willard (5) reported on variables that influenced the adsorption of sodium ions on glass, and included some data on adsorption of sodium on fused silica, steel, platinum, silver, and aluminum. Although adsorp- tion of metals on glass is well known in trace-element analysis (11), little has been reported on the adsorption of metal ions on metals. This paper reports a study of certain variables that influence copper adsorp- tion on, and desorption from, stainless steel surfaces. The data presented-- only part of those obtained in this study--are similar to and consistent with results of other experiments that are not reported. Those presented were selected t~ illustrate the influence of some important factors.