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Metals in Dairy Equipment

Corrosion Caused by Washing Powders, Chemical Sterilizers, and Refrigerating Brines

Blue Valley Creamery Company, Research Laboratories, Chicago, Illinois

ABSTRACT

The resistance to corrosion of 19 different metals, plated metals, and metallic alloys in solutions of washing powders, chemical sterilizers, and refrigerating brines was studied. The metals consisted of aluminum, manganese aluminum alloy and aluminum plated alloy; copper, nickel, Monel Metal, and nickel silver; tin, tinned copper, and tinned iron; iron, zinc, and galvanized iron; and ordinary chromium steels in the form of Ascoloy and Enduro, and a chromium-nickel steel in the form of Allegheny Metal. The metals used consisted of strips which were immersed in the solutions studied to the extent of one-half of the length of the strips. At the conclusion of each series of tests the weight losses and appearance of the strips and the color and precipitate of the liquids were noted. The results of these tests were briefly as follows:

1. In washing solutions
   1. The washing solutions used were 0.5 per cent solutions of sodium hydroxide, special alkali, sodium carbonate, Wyandotte Cleaner and Cleanser, tri-sodium phosphate, and tri-sodium phosphate containing 0.025 per cent chromate of soda. In an additional investigation the corrosion-protective effect of silicate and chromate of soda on aluminum products was studied.
      
   2. The aluminum products suffered by far the most intense corrosion in all the washing solutions used, the weight losses of the three aluminum products accounting for over two-thirds of the total weight losses of all the metals.
      
   3. The tinned copper and tinned iron products ranked next in intensity of corrosion. Their resistance, however, was much greater than that of the aluminum products, the total weight losses of the five tin-coated products being only one-seventh of those of the entire 19 metals. Tin, itself, showed varying corrosion, but as a whole it resisted attack better than the tin-coated products.
      
   4. Of the remaining metals the chromium-nickel steel, Allegheny Metal, and the ordinary chromium steels, Ascoloy and Enduro, proved most resistant, in fact, these three chromium steel alloys were practically immune to the action of the washing solutions. Nickel and Monel Metal also showed only negligible weight losses and withstood visible corrosion and tarnishing well. Nickel silver, while not suffering appreciable weight losses, tarnished considerably in some of the washing solutions, while the corrosion of copper, iron, galvanized iron, and zinc was decidedly greater but not as severe as that of the tin-plated products.
      
   5. In the case of the aluminum products, corrosion in alkali washing solutions was avoided by treating sodium carbonate with a very small amount of silicate of soda. Thus, a 0.5 per cent solution of sodium carbonate containing 0.05 per cent of sodium silicate eliminated the weight losses of the aluminum products in this washing solution entirely.
      
   6. In a similar manner the corrosion of tin-plated copper and tin-plated iron in alkali washing solutions may be reduced to negligible figures by chromate treatment of tri-sodium phosphate. This was accomplished by the use of 0.5 per cent solutions of tri-sodium phosphate containing 0.025 per cent chromate of soda. These facts are important in that they assist in solving the difficult problem of preserving the tin coating in milk and cream shipping cans, pasteurizing vats, and other tinned surfaces in milk plant equipment. They further emphasize the short life of the tinned surfaces when caustic and other strongly alkaline washing solutions are used.
      
   
   

1. In chemical sterilizers
   1. The chemical sterilizers used were solutions of sodium hypochlorite, Diversol, and chloramine-T.
      
   2. In solutions made up with approximately equal content of available chlorine, the sodium hypochlorite was by far the most corrosive of the three chemical sterilizers used.
      
   3. Diversol, on account of its alkaline properties, proved very destructive to aluminum and is, therefore, not suitable for use on aluminum equipment. However, it was the least corrosive sterilizer in the case of all the other metals and its effect on the tin-plated products was relatively slight.
      
   4. Chloramine-T also showed far less intense action on tinned products than sodium hypochlorite but its corrosive effect was somewhat greater than that of Diversol.
      
   5. Allegheny Metal and Enduro showed no visible corrosion in any of the chemical sterilizers, and the attack on Ascoloy, nickel, Monel Metal, and nickel silver was but slight.
      
   6. In neutralizing lime the aluminum products were very severely affected and the presence of oxidation products was evident. Zinc and galvanized iron also suffered considerable corrosion and weight losses. The attack on the remaining metals was slight.
      
   
   

1. In refrigerating brines
   1. Both neutral sodium chloride brine and neutral calcium chloride brine were used. Portions of these two brines were also made alkaline with caustic soda and still other portions were treated with sodium chromate and sodium silicate, respectively. In addition a series of metal strips was exposed to successive treatment with steam, water, brine, and air.
      
   2. Nickel and Allegheny Metal resisted the corrosion practically completely. In only two cases did these metals show any attack whatever and then only to a slight extent. This was in the case of the combined treatment with steam, water, brine, and air, and in the silicate-treated sodium brine. But even here these two metals were more resistant than any of the others.
      
   3. Nickel and Allegheny Metal were closely followed by Enduro, Ascoloy, Monel Metal, nickel silver, and tin, all of which showed only slight attack in most of the brines.
      
   4. Copper and the tinned iron and tinned copper products showed considerable corrosion. This was especially true of the tinned products in the alkaline brines.
      
   5. The aluminum products withstood corrosion to a marked degree in all brines except in the alkaline brines in which they were severely attacked.
      
   6. Iron, galvanized iron, and zinc suffered heavy weight losses in all the brines and showed very poor resistance to corrosion.
      
   7. With reference to the comparative corrosiveness of the individual brines, our results indicate that, in general, the action of the sodium brines was more severe then that of the calcium brines; that chromate treatment affords the best retardation of corrosion, and that in the case of some of the metals, an alkalinity of 0.05 per cent intensified corrosion. This was particularly true of the aluminum, tin, and zinc products.