the corrosion of aluminium cooking utensils

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30 TINKLER AND MASTERS CORROSION OF ALUMINIUM COOKING UTENSILS The Corrosion of Aluminium Cooking Utensils. BY C. KENNETH TINKLER, D.Sc., F.I.C., AND HELEN MASTERS, B.Sc. IN the Lancet of January 4th, 1913, an account was given of some work carried out in the Lancet laboratory entitled “Some Kitchen Experiments with Aluminium.” The general conclusion was reached that any suspicion that aluminium may communicate poisonous qualities to food in the process of cooking may safely be dismissed.” Although, as a result of experiments carried out since 1913, there appears to be no reason to doubt the correctness of the above conclusion, yet in view of the greatly extended use of aluminium for cooking purposes, an account of some experi- ments we have made with aluminium cooking utensils may be of interest. Accounts of experiments of various other investigators on the corrosion of aluminium are to be found in the Journa2 of the Institute of Metals. It is well known to those who make use of such vessels for cooking purposes that in certain cases extensive corrosion appears to have taken place, as indicated by the discoloration produced on the vessel. Thus when tap water is boiled in an aluminium saucepan a very dark stain is often produced, which is, however, much more pronounced in some cases than in others. The stain produced varies with the nature of the water and of the aluminium employed. Stains of different intensities are also often obtained with the same vessel and the same kind of water under different conditions of heating, and in some cases the stain may be partly masked owing to the deposition of calcium carbonate when hard water is boiled. It is also apparently well known that this stain is removed if the vessel is subsequently used in an operation in which an acid liquid is employed, as for example in stewing fruit, and it can also be removed if a strongly alkaline liquid is heated in the vessel, or by means of an abrasive. If water containing substances in solution or suspension is heated in an aluminium saucepan it is often found that the stain produced is much less pro- nounced than if water alone is heated in it. Thus if an egg is boiled’’ in water in such a vessel a dark stain may be produced on the metal, whereas if the same vessel is used for “scrambling” an egg no stain is produced. It would be expected that if the egg were in any way connected with the apparent corrosion the attack would be more pronounced in the second case. Since, however, aluminium may be dissolved with or without blackening, it is impossible simply from the appearance of the saucepan to decide in which case greater corrosion has taken place. THE NATURE OF THE DARK STAIN.-There is apparently no doubt that the dark stain is due to the presence of impurities in the aluminium, the chief of which is iron; and that by the action of an alkaline water an extremely small amount of Published on 01 January 1924. Downloaded by University of Chicago on 28/10/2014 22:24:02. View Article Online / Journal Homepage / Table of Contents for this issue

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Page 1: The corrosion of aluminium cooking utensils

30 TINKLER AND MASTERS CORROSION OF ALUMINIUM COOKING UTENSILS

The Corrosion of Aluminium Cooking Utensils. BY C. KENNETH TINKLER, D.Sc., F.I.C., AND HELEN MASTERS, B.Sc.

IN the Lancet of January 4th, 1913, an account was given of some work carried out in the Lancet laboratory entitled “Some Kitchen Experiments with Aluminium.” The general conclusion was reached “ that any suspicion that aluminium may communicate poisonous qualities to food in the process of cooking may safely be dismissed.”

Although, as a result of experiments carried out since 1913, there appears to be no reason to doubt the correctness of the above conclusion, yet in view of the greatly extended use of aluminium for cooking purposes, an account of some experi- ments we have made with aluminium cooking utensils may be of interest. Accounts of experiments of various other investigators on the corrosion of aluminium are to be found in the Journa2 of the Institute of Metals.

It is well known to those who make use of such vessels for cooking purposes that in certain cases extensive corrosion appears to have taken place, as indicated by the discoloration produced on the vessel. Thus when tap water is boiled in an aluminium saucepan a very dark stain is often produced, which is, however, much more pronounced in some cases than in others. The stain produced varies with the nature of the water and of the aluminium employed. Stains of different intensities are also often obtained with the same vessel and the same kind of water under different conditions of heating, and in some cases the stain may be partly masked owing to the deposition of calcium carbonate when hard water is boiled.

It is also apparently well known that this stain is removed if the vessel is subsequently used in an operation in which an acid liquid is employed, as for example in stewing fruit, and it can also be removed if a strongly alkaline liquid is heated in the vessel, or by means of an abrasive.

If water containing substances in solution or suspension is heated in an aluminium saucepan it is often found that the stain produced is much less pro- nounced than if water alone is heated in it. Thus if an egg is “ boiled’’ in water in such a vessel a dark stain may be produced on the metal, whereas if the same vessel is used for “scrambling” an egg no stain is produced. It would be expected that if the egg were in any way connected with the apparent corrosion the attack would be more pronounced in the second case. Since, however, aluminium may be dissolved with or without blackening, it is impossible simply from the appearance of the saucepan to decide in which case greater corrosion has taken place.

THE NATURE OF THE DARK STAIN.-There is apparently no doubt that the dark stain is due to the presence of impurities in the aluminium, the chief of which is iron; and that by the action of an alkaline water an extremely small amount of

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Page 2: The corrosion of aluminium cooking utensils

TINKLER AND MASTERS: CORROSION OF ALUMINIUM COOKING UTENSILS 31

aluminium is removed from the silrface, leaving behind the iron and other impurities which are insoluble in the alkaline water.

Iron is, of course, readily soluble in acid, so that the dark stain is removed by an acid liqui6, and no stain is produced when commercial aluminium, which contains iron impurities, is being dissolved in acid.

We have compared the amount of iron removed from a stained and from a bright aluminium saucepan by treatment with very dilute sulphuric acid, and find approximately twice as much in the case of the stained pan.

In the case of one sample of commercial aluminium which showed pro- nounced blackening with tap water, an insoluble residue amounting to 2.2 per cent. of the material was obtained when the metal was dissolved in a solution of sodium hydroxide, this residue being a finely divided black powder, whilst for the same material the amount insoluble in hydrochloric acid was only 0-38 per cent. When such a sample of aluminium is dissolved in sodium hydroxide solution no dark film is observed as solution proceeds, although one may appear at first, as in this case probably owing to the rapid solution of the aluminium, the particles of iron become detached and remain suspended in the solution. This explains the removal of the dark stain by strong alkali.

For a given sample of aluminium and different samples of natural waters the intensity of the stain appears to depend in most cases, on the hydroxyl ion concentration of the water ; the greater this concentration the darker the stain.

obtained by Friend (Trans. Chem. SOC., 1921, 932) on the protection of iron from corrosion in the presence of colloids, we carried out a number of experiments with reference to the protection of aluminium. There is apparently no doubt that the colloidal matter, which is frequently present when aluminium vessels are used for cooking food, materially lessens the amount of aluminium dissolved. Thus in one experiment the loss in weight of a disc of aluminium heated in a dilute solution of agar in tap water was only one-third of the loss in weight produced when the same disc was heated in tap water alone, and only very slight darkening was observed in the first experiment. The non-formation of a stain when an egg is “scrambled” in an aluminium saucepan may also be due to the presence of colloidal matter. Some results on the protection of aluminium by colloids are given by Friend (Zoc. cit . , 1922, 468.)

We have found that it is possible to prevent the discoloration of aluminium by tap water. If a solution of potassium dichromate is boiled in an aluminium saucepan no blackening takes place; in fact, aluminium so treated shows a slight gain in weight and is rendered ‘‘ passive.” Treatment with tap water does not now produce a stain.

To illustrate this action, a small quantity of a solution of dichromate is hea.ted for some time in a bright aluminium saucepan, the pan is then washed out, filled with tap water and heated. A well-defined stain is obtained above the original level of the dichromate solution, but the part previously covered by the solution remains quite bright.

THE PROTECTION OF ALUMINIUM FROM CORROSION.-In view Of the results

The effect, however, is not permanent.

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32 NOTE

In connection with the cleaning of aluminium, reference should be made to a paper by Seligman and Williams (J . Inst. MetaEs, 1922, 297; ANALYST, 1922, 47, 493). They find that the attack of an alkaline solution on aluminium is con- siderably retarded in the presence of sodium silicate, and that commercial preparations sold for cleaning the metal usually contain sufficient sodium silicate for this purpose.

CHEMICAL LABORATORY, HOUSEHOLD AND SOCIAL SCIENCE DEPARTMENT,

KING’S COLLEGE FOR WOMEN, KENSINGTON, W. 8.

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