SeptembPr, 1931
ISDUSTRIALAiVD ENGINEERlNG CHEMISTRY
999
Corrosion of Metal Articles in Storage Influence of Alkaline Surface Films' W. E. Cooper 163 WOLVERHAMPTON ROAD,QUINTON,BIRMINGHAM, ENGLAND
It is shown how the presence of small amounts of mixing with hot sawdust and UCH mild steel articles alkali deposited as an apparent uniform layer on metal r e v o l v i n g in s t e e l barrels. as bolts, screws, hooks, articles serves to increase their freedom from tarnishThe sawdust is then removed and other r e p e t i t i o n ing and rusting when stored in warehouses. Also, by by sieving, and the steel arproducts of the hardware inchoosing the most protective alkaline deposit, and adticles are sorted by hand and dustry undergo varied and justing and maintaining storage conditions in a state of placed in cartons for storage. indefinite periods of storage constant control, it is possible to store articles for very Upon one occasion, when in warehouses prior to dismany years without loss of luster. Costly storage loss !s a serious outbreak of rusting patch to customers. Some incurred by uncontrolled conditions seriously warrant occurred, a batch was more articles are known to remain investigation of the factors involved as described in this thoroughly dried than usual in stock for m a n y y e a r s , paper. by the same process. No imwhile others may be only temSpeculatively, it is suggested that many cases occur provement in corrosion resistporarily stored according to where articles are being dipped in grease for storing, ance resulted, but on the conthe market demand for such whereas investigation might prove that suitable alkatrary diminished. An even particular sizes or articles. line treatment and control of warehousing would remore thorough drying treatFollowing the final mechanisult in considerable saving of storage costs by utilizing ment again produced worse cal manufacturing operation, a cheaper method. results, a phenomenon which t h e articles are usually convinced the writer that the cleaned by washing, dried, and wrapped in cardboard and paper packages and trans- general conception of drying (removal of moisture) as the fundaported to a warehouse for storing. Manufacturers generally mental basis of resistance was erroneous. Careful testing of provide a dry, heated warehouse to ensure that the articles lubricants, soda, soap, sawdust, water, wrappers, and atmosretain their bright finish and luster, and are concerned with phere failed to produce evidence of corrosive agents likely to their liability to rust only from the commencement of manu- account for the trouble. This led to the beginning of the investigation of this problem. First of all, by laboratory duplifacture until final dispatch from warehouse. Being of readily corroded material, mild steel and wrought- cation of the drying tests it was established that resistance to iron articles are usually intended only for indoor or unexposed corrosion decreased with the intensity of drying. Upon conservice, although even under these conditions tarnishing sideration it appeared that during the washing operation in suds (soda-soap solution) the articles might become coated with quickly occurs. Under good warehousing conditions, articles are stored for the alkaline fluid, and that the effect of drying in sawdust many years without apparent tarnishing; on the other hand, might serve to remove the surplus solution and leave an adcases arise when only 2 or 3 days' storage may be sufficient to sorbed film of alkaline solution on the articles. This alkaline reveal active corrosion taking place for which an immediate film would be expected to have mild protective properties explanation is not readily obtainable. Occurrences of the against atmospheric corrosion. Disrupture or removal of this latter involve frequent close inspection, rejection, recleaning, film would lead to early corrosion of the metal. drying, and repacking, operations of considerable expense Nature of Film (Proof) when considering the small price of the articles; yet it is esA length of bright drawn mild steel wire (carbon, 0.12 per sential that the articles present a bright lustrous finish, which cent) was turned up on a lath into a number of test pieces is a factor seriously affecting salability. Application of an antirust finish or film of grease is often [2 inches by 2 inches (5.08 cm. by 5.08 cm.) diameter] using economically impracticable on such small articles. Thus two no cutting lubricant or fluid of any kind. After turning, the problems arise-(1) to prepare the articles in such a way as to pieces were placed with aluminum forceps on a glass grid inbe least readily tarnished and ( 2 ) to provide storage conditions side a desiccator containing phosphorus pentoxide. I n all of the following tests, one test sample consisted of twelve indisuch as to give immunity from corrosion. Omitting, for the present, consideration of applied grease vidual test pieces or articles, and in every instance a t least f3ms or other protective finishes generally known, it is es- seven of the pieces behaved identically. In the case of samsential that the steel articles present a smooth, dry, and ples taken from stock and from manufacturing operations, care lustrous surface. Having obtained this, the articles should be was taken to select batches of one dozen made of the same carefully packed in cardboard or paper cartons and deposited material and produced a t the same time under identical on shelves in the warehouse. Where control of the ware- conditions. The manufactured articles themselves consisted house atmosphere is available, special ventilating apparatus, of flat-head woodscrews, rivets, etc., of similar superficial air filtration, and thermostatic regulation are in operation, area to the dry-turned test blanks. Reference to the respecsince it is realized that pure clean dry and even warm air tive samples used in these tests is as follows: (I) blanks, favors immunity from tarnishing. Close control of the test pieces turned up dry as previously described; ( 2 ) treated wrappers and their testing for acidic and corrosi\e mineral blanks, test pieces as (1) but washed and dried as in the usual constituents are also essential features of successful storage. manufacturing processes; (3) machined, finished products After the final machining operation, a practice commonly fol- taken direct from final machine operation without washing lowed is to immerse the articles in a hot solution of soda ash treatment, etc.; and (4) stock articles, finished products taken and soft soap in water, in order to remove acquired grease and from the warehouse. dirt. This is followed by drying and finishing the articles by Samples of blanks, treated blanks, and stock were weighed and
S
1
Received February 26, 1931.
dried in an oven filled with nitrogen until they were of constant
1000
I S D U S T R I A L ALVDENGINEERING CHEMISTRY
ultimate weight. The loss in weight of the treated blanks and stock was roughly one hundred times the loss sustained by the dry turned blanks. Four rows of small test glasses of the same size were carefully cleaned and dried and an equal quantity of indicator solution put into each. The solution consisted of a 1 per cent solution of B. D. H . universal indicator in conductivity water. By means of a superimposed rack from which the test samples were suspended by silk threads, they were all simultaneously immersed in the indicator solution. The treated blanks and stock instantly changed the color of the solution to a violet color (pH 10.0, approximately), while the blanks only slowly changed the indicator solution toward alkalinity. This test was repeated with various sizes and shapes of stock samples with the same results. Samples of blanks, treated blanks, and stock were placed in separate dried Pyrex flasks and a quantity of distilled water added to each. A few cubic centimeters of purest hydrochloric acid were added and the mixture allowed to stand for about half an hour. The solutions were then filtered and evaporated to dryness in platinum dishes, and tested chemically and spectroscopically. With both the treated blanks and stock samples, positive evidence of the presence of sodium compound was obtained. Samples of blanks and treated blanks were exposed to indoor and outdoor atmospheres and daily observed for corrosion by visual inspection and weighings. In the exposure tests, shielded wooden cabinets were used to prevent direct or irregular currents of dust, rain, etc., impinging on the samples. The blanks began to tarnish and then rust almost immediately, while the treated blanks were only slowly affected but, when corrosion had set in, rusting progressed rapidly. Samples of stock and treated blanks were placed in a desiccator containing phosphorus pentoxide for one day; a similar set of samples was dried in an oven containing nitrogen a t 110' C . ; and another similar set was taken when the previous samples were ready after drying. All these samples were submitted t o the indoor and outdoor exposure tests, and those which had been subjected t o drying processes gave earlier evidence of corrosion than the undried.
The foregoing tests appear to prove definitely the existence of an alkaline film as has been suggested, and the possession by the film of mild protective properties against corrosion. Essentially the protective film consisted of an adsorbed layer of washings suds (soda-soap solution) which was no doubt somewhat concentrated upon the metal surface during the drying operation. During the exposure tests it was observed that tarnishing and rusting occurred evenly over a smooth surface, indicating somewhat uniform distribution of the alkaline layer. Being of small dimensions and weak properties, this &-Iwas readily influenced by external and adjacent conditions. First of all, different composition of sodasoap solutions would not be expected to yield films of similar protective nature. Again, the film, being of a soluble nature, would be very readily affected by atmospheric conditions, and adjacent materials, such as wrapping paper, might be expected to exert influence on .the general stability and protective properties of the film. Tests Showing Effect of Influencing Factors
COMPOSITION OF Frm-In these tests the sawdust was freshly obtained from yellow pine wood, and bested for freedom from acidic materials. The washing and drying tests were similar to the manufacturing operations used a t the factory being investigated. After immersing the samples for five minutes in soda-soap solutions maintained a t 90" C. (losses by evaporation were controlled by means of a condenser) and then shaking in proportional weights of sawdust for 10 minutes in revolving small iron drums, the samples were separated from the sawdust by sieving. Handling was by means of clean steel forceps, and the following materials were used : Soda ash, tested a t 58 per cent total alkali, traces only of chlorides, sulfates, etc., found. Soft soap, pure commercial variety about 40 per cent total fatty acid, with 1.5per cent free alkali. Water: distilled.
Vol. 23, No. 9
Solutions were mzde covering the following ranges of composition : (1) 0-10 per cent soda ash alone, as grams per 100 cc. water. ( 2 ) 0-10 per cent soft soap alone, as grams per 100 cc. water. (3) 0-10 per cent soda ash, to which 0-10 per cent soft soap was added, as grams per 100 cc. water.
Some of the solutions, particularly the more concentrated ones, were impractical, owing to separation, and only when the latter were clear solutions a t operating temperature (90" C.) were they used; otherwise they were rejected. Samples of blanks, machined, and stock, the last mentioned being recently finished products from a section of the factory in which no trouble was experienced, were treated with all the ranges of solutions. They were then dried and submitted to the exposure tests. At the same time samples of good stock articles directly from a section of the factory in which no trouble was experienced were likewise submitted to the exposure tests with the previously mentioned test samples. Several of the individual good stock, blanks, and machined treated test pieces were sealed in test tubes containing phosphorus pentoxide, and these were used as standard brightness comparators-i. e., to note the tarnishing and rusting of the test pieces. The exposure test comprised the following, using shielded cabinets in each case: laboratory atmosphere, indoor workshop atmosphere, and outside atmosphere. Using the standard brightness comparators, the relative rate and order of corrosion was followed by daily inspection and recording. As indicated previously, rusting proceeded a t a rapid rate as soon as the appearance of brown rust was observed after the completion of the tarnishing stage. All three exposure tests produced practically the same relative order of corrosion of the test samples which had been treated with the ranges of solution previously mentioned. It was concluded from the total results of these exposure tests that the following range of solution was the most effective under the circumstantial conditions of the tests: Soda ash, 0.05-0.5 gram per 100 cc. water Soft soap, 0.25-3.0 gram per 100 cc. water
The previous tests were then repeated with the same range of washing solutions and, by more carefully noting the relative order of corrosion, a solution composition was obtained which appeared to be the ideal and which is hereafter referred to aa the standard-solution suds: Soda ash, 0.2 gram per 100 cc. water Soft soap, 1.5 grams per 100 cc. water
It is to be particularly noted that the results obtained apply only to the conditions and circumstances prevailing in the fsctory a t the time this investigation was made, and no suggestion of general application is intended. HARMFUL CONSTITUENTS AND VARIATION IN CONTACT MATERIALs-It is naturally to be anticipated that wrapping papers having acidic reaction or containing f l i n g materials of corrosive nature, such as sulfites, chlorides, etc., would have adverse effects upon the wrapped articles. This factor had been realized some time before, and at the factory all types of wrappers were frequently tested, and were generally found to have a slight alkaline reaction (about p H 9). Cases of corrosive paper had been previously detected by chemical analysis, and the alkaline film was found to have no material protective action against such corrosive wrapping. The main factor associated with wrappers as well as with the surrounding atmosphere is that of humidity. HUMIDITY AND DRYING Ac~lo~-Samples of treated blanks (treated in the standard-solution suds) and of stock were submitted to the following atmospheric conditions for one week;
INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1931
1001
at the same time sheets of wrapping paper were also submit-
cent was obtained. These tests, as well as those previously described, illustrate the important action that humidity call exert on wrapping papers and, in consequence, on the con(1) Desiccator containing phosphorus pentoxide, a t 60" F. tained articles. (15.56' C.). (2)0 Desiccator containing saturated solution of CaCls 6H20, SHAPEOF ARTICLES-~~S previously stated, the blanks conat 60 F. (15 56' C.), relative humidity, 35 per cent. sisted of smoothly turned cylindrical pieces of steel, while the (3) Warehouse, a t 60' F. (15.56' C.), relative humidity, stock articles mere wood screws, rivets, etc., having similar 60-70 per cent. superficial area. I n the exposure tests it was observed that (4) Desiccator containing saturated solution CaS0a.5Hs0, a t 60 F. ( l a 56" C . ) ,relative humidity, 99 per cent. the smooth surfaces-i. e., the heads and shanks of the finished products-suffered corrosion first, usually in a regular manner. First of all it was observed that under conditions (l), (2), Such places as depressions inside the threads were affected and (3) the samples were little, if any, tarnished by the week's by corrosion last, apparently because of the presence of a larger treatment but, in the case of (4),tarnishing and rusting were amount of alkaline fluid; on the other hand, sharp corners and visible. As in previous tests, standard brightness comparaedges suffered early attack, owing to lack of protective film. tors were used in judging the degree of tarnishing. After the It is also interesting to note that those portions of wrapped week's treatment the samples were taken out and submitted to articles in direct contact with the wrapping paper were the exposure tests as before. The results showed the following parts first affected. This was particularly noted during the order of resistance to corrosion, best to worst: exposure tests in connection with humidity and dehydration. An additional investigation carried out with the cutting 1-Samples exposed to treatment (3) 2-Samples exposed to treatment (2) fluid used in the machining operations of manufacture showed 3-Samples exposed to treatment (4) that articles initially in contact with dirty fluid were less resist4-Samples exposed to treatment (1) ant than articles produced with clean fluid, although given Samples of stock and treated blanks (using standard identical washing and d y i n g treatment. This is no doubt solution) were enclosed in the various treated wrappers and due to the fact that contamination of the original metal surface placed in the marehouse for one week, taken out and wrapper with dirt is not wholly removed by washing. discarded, and the samples submitted to the usual exposure Factors Governing Storage Problem tests. The following order of resistance was obtained: ted to these conditions for the same length of time:
1-Samples 2-Samples 3-Samples &Samples
wrapped wrapped wrapped wrapped
in in in in
papers papers papers papers
treated treated treated treated
by by by by
(3) (2) (1) (4)
Upon consideration of the results of these investigations, it appears that storage of mild steel articles for a length of time requires coating the articles with a good protective film, and adjusting and maintaining conditions of handling and storage such that the film is apparently in a state of stability or undisturbed equilibrium. Frmt-Upon immersing the articles in a nearly boiling solution of soda-soap suds, they acquired a certain amount of latent heat which was liberated when they were immediately put into the sawdust for drying. This liberation of latent heat resulted in a concentration of the washing solution on the metal surface, resulting in the alkaline film layer. In the particular experiment referred to the resisting power of sodium carbonate was enhanced by the addition of soap, which no doubt increased the stability of the film. Seither substance individually was as protective as when mixed, and from the experiments described, was derived a formula of maximum protection, consistent with the circumstances and conditions existing at the factory at the time of this investigation. Too concentrated solutions were not effective and were, in fact, the actual cause of trouble experienced at that time. In general, no doubt a definite alkalinity as pH is necessary, while physical characteristics of the film determine its stability. FACTORS AFFECTINGSTABILITY OF FILM-In connection with both atmosphere and wrapping paper it has been indicated that drying or hydrating action seriously affects the duration of protection from corrosion. Dehydration by virfue of low humidity or relative unsaturation of the wrapping paper can result in actual dehydration and disruption of the protective film, On the other hand, under humid conditions, dilution of the film occurs with possible carriage of air, carbon dioxide, etc., toward the metal surface, thereby lessening the resistant power of the film. It appears that for longest protection from corrosion a state of physical and chemical equilibrium is necessary for the complete system:
These tests appear to prove that a drying atmosphere has the effect of dehydrating and thereby disrupting the mild protective alkaline film. I n a similar manner a wet atmosphere dilutes the film and renders the base metal readily accessible to atmospheric corrosion. These influences are exerted both directly and indirectly via the wrapping. ATMOSPHERICIMPuRITIEs-In industrial localities the presence of sulfurous and other deleterious impurities of the atmosphere naturally affect successful storage of articles. In this particular warehouse a regulated system of ventilation existed with forced intake, air filtration, and humidifying control, by which it was possible to regulate the air so as to obtain and maintain it a t about 60" F. (15.56' C.) and relative humidity of 60-70 per cent. Check tests were periodically taken against other impurities, but actually no remedying treatment was required in this instance although, elsewhere in the neighborhood of severe industrial fumes, further treatment would have been necessary. Dus-The influence of dust from wrapping papers and other materials in the warehouse was shown to affect the corrosion of articles by the fact that shielded samples various kinds corroded more readily than unshielded ones when submitted to warehouse conditions. I n consideration of the results obtained it was decided to use wrapping paper of a strong and smooth variety which would not readily shed dust particles and which was not open pored. Likewise measures were taken to minimize the raising of dust of any description. MOISTURECONTENT O F WRAPPlKG PAPERS-sELmpleS Of Kraft and Rope-Strip wrapping papers showed, on testing, a moisture content varying from 8.0-12 per cent as stored in the different paper storehouses. Upon exposing samples of these papers to the warehouse atmosphere, the papers singularly assumed a similar moisture content (8.9-9.1 per cent). metal/film"wrapper$atmosphere Exposure to laboratory atmosphere likewise gave similar moisture content (10.8 to 11.5 per cent); while in a desiccator and it is necessary to consider each of these factors in turn containing saturated zinc chloride solution (relative humidity, with respect to obtaining a state of final equilibrium. In the 10 per cent), a practically constant moisture content of 4.8 per previous tests these factors were considered on rhe lines of inlof
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vestigation as described, and in consequence the following ideal conditions were resolved: (1) Washing the articles in a solution of 0.2 gram soda ash and 1.5 grams soft soap per 100 cc. water. (2) Maintaining warehouse a t constant humidity and temperature-i. e., relative humidity, 65 per cent, temperature, 60' F. (15.56' C.). (3) Preserving wrapping papers a t same constant humidity and temperature as warehouse. (4) . Constant control of all materials and agents for possible impurities and variations likely to cause corrosion.
It should be noted that all conditions must be maintained strictly constant, otherwise the supposed equilibrium is disturbed.
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Limitations
Naturally this form of protection is of a relatively small order and applies only to indoor storage conditions, particularly because of the solubility of the film. Also, the small magnitude of the film is well manifest when one considers that mere handling of the articles, especially by a person with perspiring hands, is quickly shown by the fact that corrosion sets in very early a t the parts so handled. Articles with very sharp edges and corners, such as deeply cut close-threaded bolts, etc., are not successfully protected in stock by the usual alkaline washing treatment and necessitate dipping in thin mineral oil, lanolin solution, or similar greasings. This appears to be associated with the fact that, as stated previously, sharp edges of metal are not coated with as efficient an adherent layer of alkali as the smoother surfaces.
Dependence of Reaction Velocity upon Surface and Agitation 11-Experimental Procedure in Study of Surface' A. W. Hixson2 and J. H. Crowell* DEPARTMENT OF CHEMICAL ENGINEERIXG, COLUMBIA UNIVERSITY, NEW
YORE,
N Y.
The cube root law has been experimentally verified the solution was employed. throughout a wide variety of conditions in the study fication of t h e cube For the lower intensities of of heterogeneous reaction kinetics in the solid-liquid root law in a solidagitation this method is undissolution system. Its experimental verification in liquid dissolution system has suitable, but it is sufficiently the solid-gas system (naphthalene-air) has been made, been accomplished u n d e r accurate for the higher intenand certain variables have been studied. various types of a g i t a t i o n . sities. For the verification of The study of a solid-liquid chemical system (alumIts special cases were investit h e law o n l y t h e d i r e c t aqueous ammonia) has been described in which the gated and shown to a p p l y method was employed. effect of protective coatings upon the operation of the law under many different experiAs both of these methods was satisfactorily explained. m e n t a l conditions. It was have long been used in studies The work of other investigators upon certain autofound that the cube root law of the rates of d i s s o l u t i o n , catalytic reactions in solid-solid systems has been corwas very general in its applicatheir employment seemed to related to that contained in this paper. tion, and a study of several be j u s t i f i e d . T h e d i r e c t other types of agitation sysmethod is obviouslv to be oretems was mad-. g., the solid-gas and solid-liquid chemical ferred whenever its use is possible, since the rate i f distribution does not need to be considered. I n all cases, unless othersystems. wise stated, the liquid in contact with the surface of the parVerification of Cube Root Law ticle was in motion. Complete stagnation was not desired, as DIRECTMETHOD-In most cases the change in weight of the this would be the condition for diffusion. Also, unless differsolid was determined by removing it from the liquid and actu- ently stated, the temperature used was 20" * 0.2' C. GESERALLAW,EQUATION 4-The general law may beillusally weighing it after each time interval. Considerable care was taken to maintain standard time intervals for its inser- trated by the following experiment, whose results may be tion and withdrawal, and these were kept negligible compared found in Table I and Figure 1: with the solution intervals. It was soon found that by the ( a ) Conditions-Naphthalene, alcohol (95 per cent), uniform proper choice of materials, concentration, agitation, and free rotational agitation. amount of surface, the rate of solution could be varied over ( b ) Method and Apparatus-The container was a cylindrical weighing bottle, 4.1 cm. in diameter and 5.1 cm. high, with a LI wide range so that there was no necessity for working of 50 cc., stirred with a small L-shaped single paddle with a rate so fast that sensible errors would be introduced capacity glass agitator of 150 r. p. m. The particle was a common ellipin the insertion and removal of the solid. When the solid was soidally-shaped naphthalene mothball such as can be purchased slightly volatile, as in the case of naphthalene, it was always at any drugstore. It had evidently been molded by tightly weighed and handled in tightly stoppered weighing bottles. pressing the ground particles together while they were slightly There were no airholes or irregularities in it a t the start, INDIRECT METHoD-when the distribution of the dissolved warm. nor did any appear during the run. It was well rinsed off with solid was sufficiently uniform so that it was apparent that the alcohol before using, in order to remove surface grains, etc. loss in weight could be computed from the concentration I t s maximum length (L)was 21.5 mm., and its width ( W ) was change, the method of withdrawing and analyzing samples of 17.5 mm. Ratio L:W = 1.23. Weight ( W O )= 3.5324 grams.
HE experimental veri-
T
Received February 21, 1931. From a dissertation presented by J. H. Crowell to the Faculty of Pure Science, Columbia University, in partial fulfilment of the requirements for the degree of doctor of philosophy, June, 1930. 1 Professor of chemical engineering, Columbia University. a Present address, The Selden Company, Pittsburgh, Pa. I
Area = 520 sq. mm. (approx.). Amount of alcohol used, 28.25 grams (35 cc. a t 20" C.) which required a weight ( w , ) of 1.8520 grams of naphthalene for saturation. The vessel was kept covered Note-The solubility of naphthalene in 95 per cent alcohol was determined experimentally. The following data were obtained by use of the synthetic C., 20.1760 grams of alcohol dissolve 1.0 gram: at 14.8" method: a t ll.Qe
