PATRICK A N D W A L K E R O N T E S T I N G GALVANIZED IRON. tion. It should be noted t h a t this same treatment bf the residue may be necessary with some ores even when using the hydrofluoric method of attack. The fact t h a t the method as above given does not eliminate the silica is not really a drawback, since the silica is obtained in a form which does not seem t o clog the filter much. Attacking the ore b y fusing with sodium peroxide was also tried and i t proved fairly successful. Were i t possible t o procure iron crucibles free from manganese this would be the ideal method. Since that seems t o be impossible a t present, I had t o use nickel crucibles. It should be noted that some nickelcontains traces of manganese, but not, I believe, as a rule. I n using this method, I gram of ore is fused with about six grams of sodium peroxide a t as low a heat as possible. A minute or two in liquid fusion is all t h a t is necessary. The crucible and contents are treated with water in a covered beaker, and when the action is over the crucible is removed and one-third the volume of concentrated nitric acid added. The solution is boiled until all hydrogen peroxide is decomposed and everything dissolved but some manganese dioxide which has been precipitated. Ferrous sulphate is added until the manganese is reduced, the lower oxides of nitrogen are boiled off, and the analysis is finished as usual. The crucible is considerably attacked, but if the heat is kept low a dozen fusions may be made in a crucible before it is unfitted for further use. The color, due to the nickel, tends t o obscure the end point with permanganate somewhat, but after a little practice the point can be readily seen.
[CONTRIBUTION FROM T H E
239
RESEARCH LABORATORY OF MASS INST OF TECH1
A P P L I E D CHEMISTRY.
METHOD FOR TESTING GALVANIZED IRON TO REPLACE THE PREECE TEST. By
WALTER
A.
PATRICK A N D W I L L I A M
H.
WALKER.
Received February 27. 1911.
Some time ago one of us1 published a paper on the testing of galvanized and other zinc-coated iron, in which i t was shown t h a t the so-called Preece Test was unsatisfactory from many points of view, although no alternative method was shown t o be equally available. This study has been continued, and it: is now believed t h a t a satisfactory substitute can be given. The Preece Test consists in placing the piece of galvanized iron t o be tested, in a solution of copper sulphate under standard conditions, and observing the number of one-minute immersions which can be made before copper in a bright adherent form will plate out on the article. The accuracy of the test depends upon the following assumptions: first, t h a t the zinc will pass into solution and be replaced b y copper a t a definite rate, and hence the time taken t o dissolve a galvanized coating will measure its thickness; second, t h a t the galvanized coating is homogeneous and t h a t the speed of the reaction between the coating and copper sulphate is constant, that is, the rate of solution is uniform; third, t h a t when the iron base is reachedithe copper will plate out on the iron in a bright adherent film, in contradistinction to the black spongy form in which i t appears upon the zinc coating, and t h a t no bright copRer will be seen until the iron base is thus uncovered.
SUMMARY.
S T R U C T U R E O F Z I N C COATING.
Corroboration of Blair’s statement t h a t for small amounts of manganese the bismuthate is the most accurate method known. 2. When the potassium permanganate is standardized against sodium oxalate or iron, and the titer theoretically calculated, the results come out too low. A gravimetrically standardized manganous sulphate solution is the correct primary standard, but as this method is more inconvenient than the sodium oxalate method, i t is suggested t h a t pure Sorensen sodium oxalate be used, and t h a t the empirical factor 0.1656, and rzot the theoretical factor 0.16024, be used in the conversion of the sodium oxalate figure t o t h a t for manganese. 3. The decomposition of ores by the hydrochloric and sulphuric acids method is suggested as being fully as accurate, more rapid and perhaps more convenient than the hydrofluoric and sulphuric acids method. Fusing the ore with sodium peroxide is recommended as a method suitable for refractory ores.
The correctness of the first two assumptions depends upon a homogeneous structure of the galvanized coating. Although a detailed description of the structure of the different forms of zinc-protected iron now on the market will form the subject of a later paper, we will point out a t this time t h a t this structure is such as would almost certainly preclude the possibility of the Preece Test being a valid one, The coating of a n ordinary hot dipped sheet or wire is made u p of three well defined parts, namely, the iron base or foundation upon which the coating rests, then a layer of a n iron-zinc alloy, and next t o this the layer of zinc. The thickness of the alloy depends upon the temperature of the galvanizing bath, the length of time the iron is in contact with the molten zinc, and upon the flux which is used. I n a previous paper, one of usz stated that this layer of alloy was electro-negative t o the iron. This was a n error introduced b y the use of measurements of absolute potential, and which we herewith desire t o correct. The alloy is much less electro-positive than zinc, but is not electro-negative. Since the rapidity with which the zinc or zinc alloy will pass into solution, and a n equivalent weight of copper be precipitated in its place, is a function of the difference of potential between the two metals, i t will be
I.
UNIVERSITY OF
MINNESOTA.
1 2
Walker, Proc. A m SOC.Testzng Materaals, 9, 431. LOC.cd., p. 432.
T H E J O U R N A L dF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y . seen t h a t the rate of solution must of necessity change as we pass from the zinc t o the alloy. The so-called sherardized iron and some kinds of electro-galvanized iron consist so largely of a n iron-zinc alloy t h a t any test or measurement based upon a n assumed uniformity in rate of solution is liable t o grave error and untrustworthy. S P E E D OF S O L U T I O N . '
When a piece of clean zinc is placed in a reasonably concentrated soIution of copper sulphate, the rate at which zinc will pass into solution and a n equal number of copper ions be precipitated as metallic copper will depend upon the concentration of the copper ions in the solution in the immediate vicinity of the metallic zinc. If for any reason the solution become depleted in copper ions, the speed of this interaction will decrease. This is just the condition which obtains when a one-minute immersion is used in the Preece Test. For the first few seconds the reaction is very rapid, but as the spongy copper forms on the surface of the zinc it becomes more difficult for the zinc ions formed t o get away, and fresh copper ions t o reach the metallic zinc, so t h a t a t the end of a minute the reaction has practically ceased. When the sponge of precipitated copper is removed a n d the test piece replaced in the solution action again begins vigorously b u t again falls off. It can easily happen, therefore, t h a t the iron base will be practicallyexposed at, say, theend of the second minute, a n d yet no bright copper will be seen until the sponge is removed at the end of the third minute. The test will thus be classed as a three-dip piece, while in reality i t is b u t a trifle over two-dip, and a n error of 33 per cent. is thus introduced. I n fact, we have seen galvanized iron which showed areas of no coating whatever. The Preece Test would indicate such wire a s one dip when in reality i t was zero-dip wire. APPEARANCE
OF B R I G H T C O P P E R .
It has been proposed t o provide against the above difficulty b y using a number of ten-second immersions instead of one-minute immersions. But a new difficulty here arises. As is well known, when a piece of iron is placed.in a neutral copper sulphate solution iron passes into solution and copper plates out in a bright adherent film. The appearance of this bright copper is taken in the Preece Test as a n end point, and as a proof that the zinc has all been dissolved and the iron base exposed. The fundamental error here introduced can be easily shown b y immersing a piece of smooth spelter or sheet zinc t o a number of ten-second immersions in the Preece copper sulphate solution. I n a short' time a point of bright copper will appear which as the test proceeds will spread over the entire surface, and were the specimen being tested a galvanized article instead of one of pure zinc, the totally erroneous conclusion would be reached t h a t the iron base had been uncovered, and the test concluded. This layer of bright copper is formed more easily on the zinc-iron alloy than i t is upon pure zinc. Care must be taken, therefore, in carrying on the Preece Test t o avoid conditions
April,
I ~II
which would subject the test specimen to a dilute copper sulphate solution, such as obtains when the specimen is frequently rinsed with water, and t o avoid a burnishing or rubbing action when the specimen is cleaned. We attempted t o show the change in rate of solution as we passed from the zinc layer, t o the zinc-alloy layer, b y plotting the loss in weight of coating against the time of immersion. Owing to this irregular tendency of the copper t o remain attached t o the coating and t o more or less protect the coating underneath from solution, the results were altogether misleading. Although this abnormal appearance of bright copper is much less liable t o occur when oneminute immersions are employed, no dependence can be placed upon the end point as thus obtained. C A U S T I C SODA M E T H O D F O R T E S T I N G C O A T I N G .
We have already pointed out t h a t hot concentrated caustic soda will dissolve the zinc coating when placed in contact with iron. Under some conditions an alloy very high in iron is left upon the iron base. Altogether the method is not a convenient or a rapid one, and therefore we desire t o propose the following substitution for the Preece Test, devised by Mr. Patrick. BASIC
LEAD ACETATE METHOD.
When a zinc-coated iron article, free from the products of corrosion, is placed in a basic lead acetate solution at ordinary temperature, the coating passes into solution and an equivalent amount of metallic lead is precipitated in a loosely adherent form upon the specimen. This lead is easily removed and the coating determined by measuring either the loss in weight of the test specimen, or the lead precipitated. The reaction is retarded b y the precipitation of the lead and therefore when a heavily galvanized piece is being tested this lead must be periodically removed. Either a volumetric or a gravimetric method for determining the coating may be employed, according t o whether a balance is available or not. If the volumetric method is used, this lead must be preserved, while if the specimen be weighed the lead may be discarded, unless on account of the size of the specimen i t is found desirable to collect and weigh the lead. When the iron-zinc alloy is thus dissolved, the iron of the alloy passes into solution, and existing as basic iron acetate colors the solution red as i t oxidizes. If desirable this iron can be determined b y weighing as oxide, or b y titration with standard oxidizing solution. When the coating has all been removed and bright iron only is visible, either the lead is quantitatively determined, or the iron base is dried and reweighed. SAMPLE.
The size of sample must of necessity depend upon the material under investigation. I n the case of sheets or plates, a piece 2 by 2 inches, and for wire a piece 3 t o 6 inches is desirable, according t o the gauge. The size of the wire and sheet should not vary more than x / 6 4 inch plus or minus. The longer the test piece, the smaller is the percentage error
PATRICK A N D W A L K E R ON TESTING GALVANIZED IRON. of measuring. The specimen should be weighed if the gravimetric method described below is to be employed, to three decimal places if possible. SOLUTION.
The solution should be made up as follows: 400 grams of crystallized lead acetate are dissolved in one liter of water and four grams of finely powdered litharge added. The mixture is shaken until most of the litharge is dissolved. Any insoluble residue may be allowed t o settle to the bottom from -which the clear solution is later decanted and diluted to a specific gravity of 1 . 2 7 j a t 15.j o C. This produces a solution of basic lead acetate which dissolves zinc and zinc-iron alloy b u t does not attack the iron base. ME’THOD O F I b l M E R S I K G .
Enough of the lead acetate solution should be employed to completely cover the specimen, and, on the other hand, the specimen should be so placed in the containing vessel that the zinc surface is completely exposed to the solution. This is a matter of detail that will vary with the form of the specimen, but wire should be placed on end and sheets on a n edge, in order t h a t the zinc acetate formed may readily pass off into solution. Tall, narrow, glass cylinders are desirable for the wire, while beakers or thin glass water tumblers may be used for sheets or other forms . TIME O F IMMERSION.
The test specimens should be allowed to remain in contact with the solution for three minutes. The adherent lead is then removed from the specimen with the hands, a rubber “policeman,” or a stiff brush (into preferably another beaker or tumbler, if the lead is t o be determined). A burnishing ;action must be avoided as under some circumstances closely adherent lead may be plated out on the zinc. The specimen is then replaced in the lead acetate solution and after another three-minute immersion the lead is again removed as before. This is repeated until the bright surface of iron is exposed. Four threeminute immersions are usually sufficient. The bright iron base differs so radically in appearance from both the spongy :lead, or even lead in the form of a bright adherent film, t h a t a lack of uniformity in the thickness of the coating is readily detected. This is a n important point in testing wire. A thin place on the wire may frequently show the bright iron surface after the first immersion; we have found some specimens of galvanized wire which over considerable areas carried no coating a t all. On such places no lead is precipitated, and the bare iron easily recognized. D E T E R M I N AT10 N 0 F C 0 AT1 N G.
( A ) If a balance be employed the bright iron piece is now thoroughly washed with water, rinsed in alcohol; and dried and weighed. The difference between the first and second weighings gives the weight of coating on specimen employed. Suitable factors for converting this into weight per unit area are given later; but, for example, if exactly 6 inches
241
of wire be used, the weight of coating in grams as above determined, multiplied by 23.31, gives the weight of coating in pounds per mile of wire. This value can be readily calculated into pounds of coating per ton of wire if the weight of the wire per foot be known. If the weight of the test specimen be very large with respect to the weight of the coating, small errors in weighing invalidate the accuracy of the test. In this case i t is best t o collect the lead either on a Gooch crucible, and wash with boiled water and weigh, or to gather i t together into a lump and weigh after washing with boiled water by decantation. (E?) If no balance be available the weight of zinc per mile or per unit area can be determined volumetrically as follows: the accumulated lead from the specimen is washed by decantation several times with hot water previously boiled, until free from lead acetate. This is very readily accomplished. The metallic lead is dissolved in a small quantity of hot concentrated nitric acid, for example, two cubic centimeters. This solution is then neutralized with a slight excess of ammonia and without filtration re-dissolved in acetic acid. A standard solution of potassium chromate (K,CrO,) is prepared containing exactly 12.624 grams per liter. Potassium chromate reacts with lead acetate, giving yellow lead chromate. On the other hand, silver chromate is deep red. So long as there is any lead acetate in the solution, the potassium chromate will unite with this lead acetate, so t h a t if a little of this solution be placed on a paper containing silver nitrate no reaction will be observed. The moment enough potassium chromate is added t o entirely precipitate the lead, the first drop of excess of potassium chromate will color silver nitrate paper red and thus determine the end-point of the reaction. By measuring from a burette, therefore, the exact number of cubic centimeters necessary t o give the first indication of red with silver nitrate paper and using the following factors, the weight of zinc per unit area can be directly computed. FORWIRE. Length of test piece taken in inches. l... 2 3
Factor for converting cubic centimeters into lbs. coating per mile. ......................... 0.60
................. .................... , . . . . . . . . . . . . .0 . 2 0 ..................... 0.15 FOR SHEETS.
Size of test specimen. 1 sq.inch. 4 sq. i n c h , , ,
Factor for converting cubic centimeters into ounces coating per square foot.
..
...................... , , ,,
...
0.0213 0.0053
It is apparent t h a t working charts may easily be prepared from which the data desired can -be read off directly. The following formulae express the relationships most frequently used: Let Let Let Let Let Let
X
Y N
L S G
pounds coating per mile of wire. ounces coating per square foot of sheet. cubic centimeters potassium chromate solution used. length of test piece in inches, area of test piece in square inches. = loss in weight in grams. =
= = = =
c
T H E f O U R N A f , OR I N D U S T R I A L A N D ENGIi'1'EERISG C H E M I S T R Y .
542
x G x=-139.86 L =
--
0.60 x N L
y =
--S
5.090 x G 0.0213 x N
=-
The manufacturers of both sheets and wire publish small handbooks which may be had for the asking, giving the gauge, relation of weight to area and length and other data for further calculations. The following results obtained b y this method on three samples of No. 9 gauge wire are fairly representative of the accuracy attainable: LENGTHO F TEST SPECIMENS, THREE INCHES Sample Loss of No. 1. weight. Analysis 1 . . . . 0.1030 Analysis 2. , . . 0,1010 Analysis 3. , . . 0,1025 Analysis 4 . . . . 0.1035 Sample No. 2. Analysis 1 . . . . Analysis 2. . . . Analysis 3. . . . Analysis 4 . . . . Sample N o . 3. Analysis 1. . . . Analysis 2. . . . Analysis 3. . . . Analysis 4.. . .
Loss in weight. 0.1845
......
Lbs. coating per mile of wire. 4.81 ' 4.75 4.79 4.86
42.7 42.6 42.8
Lbs. coating per mile of wire. 8.55 8.54 8.52 8.56
Lbs. coating per ton of wire. 55.23 55.17 55.04 55.29
24.8 24.6 24.7
4.93 4.96 4.92 4.94
31.85 32,04 31.78 31.91
cc. KICrOl used.
0.1050
......
......
Lbs. coating per ton of wire. 31.07 30.68 30,94 31.39
DETERMINATION O F IRON I N ALLOY O F COATIKG.
The iron which is in combination with zinc in the coating as a n 'iron-zinc alloy will pass into solution as iron acetate and can be determined as follows: the combined lead acetate employed for the sample, together with the wash waters from the metallic lead, is heated t o boiling and the lead precipitated with a slight excess of sulphuric acid, the iron oxidized with nitric acid, and precipitated with ammonia, washed and weighed as iron oxide. Or if no balance be available, it may be washed, re-dissolved in sulphuric acid, reduced with zinc, and titrated with standard potassium permanganate solution. An idea of the amount of iron in the different kinds of zinccoated iron may be obtained b y the following results obtained from products found in the open market.
4
Iron in coating, Kind of material. Per cent. Ordinary hot dipped sheet. ....................... 2.26 Sherardized plate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 ,70 Wet galvanized (electro deposited) sheet. . . . . . . . . . . . trace Wet galvanized (electro deposited) sheet. . . . . . . . . . . 7 46 NOTES.
( I ) I n order to insure the absence of free acetic
acid, and t o eliminate any danger of the iron base being attacked, enough lead oxide is added to render the resultant solution slightly basic. (2) It is important to immerse the test specimen for periods not shorter than three minutes a t a time. Otherwise a coherent deposit of lead may be formed.
April, 1911
(3) I n the case of articles t h a t are weathered, or have been subjected t o corrosion, i t is necessary first t o remove the layer of zinc salts from the surface by immersion in dilute hydrochloric acid. I n such cases the gravimetric determination b y difference must needs be used t o obtain accurate results. (4) I n washing the metallic lead free from lead acetate, i t is necessary in very a.ccurate work to use water previously boiled to free it from oxygen and carbon dioxide. (5) I n order to obtain exact lengths of wire, it is well t o cut the specimen a little longer than desired and file t o exact measurement. (6) The presence of a deep red color in the lead acetate after a test must not be taken as indicating a large amount of iron. Traces of iron as basic ferric acetate produce a strong color. ( 7 ) If the iron is determined b y titration with potassium permanganate, i t is necessary t o first remove it from the acetic acid, as with acetic acid potassium permanganate titration is not accurate. (8) The size of the specimen which can be taken, if the volumetric method be used, is limited b y the amount of potassium chromate solution required for reacting with the precipitated lead. Three inches of wire are all t h a t are necessary, except that greater accuracy in measuring is obtained b y using a longer piece. ( 9 ) Even when employed in the same way in which the Preece Test is used, the lead acetate solution has important advantages over copper sulphate in this: t h a t a bright copper surface on the zinc cannot be distinguished from the bright surface on the iron base. I n the lead acetate solution, bright adherent lead may be precipitated, b u t cannot be mistaken for the bright iron. Hence, if i t is desired to make a rough test for uniformity of coating on a wire, one-minute dips may be employed, and the uniformity with which the bright iron appears indicates the evenness of the coating. THE FORMATION TEMPERATURE OF CARBORUNDUM. B y HORACEW. GILLETT.
Received hlarch 1. 1911.
The temperature a t which carborundum is formed has been given in the literature a t values varying from 1200' C. to 3 8 7 0 ~C.I The only values on which any degree of dependence can be placed are those of Tucker and Lampen' who give for the formation of carborundum 195oO, and of graphite from carborundum, 2220' C. Some experiments having indicated t h a t carborundum was formed a t a lower temperature led t o an investigation of the point. The furnace used in the final runs was about 2 7 1 Stansfield, The Electric Furnace, 1907, 152 ; Acheson, Electrochem. I n d . . 1, 332 (1903); U. S. Pat., 723,631;Dunlap, Electrical Rariew, 62, 702 (1903);Scott, Jahrbuch d . Electrochemie, 12, [23 734 (1903); J . SOC. Chem. I n d . , 24, 501 (1903);Proc. Faraday SOC., April 4, 1905;Kunz, T r a n s . 7 , 249 (1905);Pring, J . Chem. Soc.. 93, 2104 (1908); A m . Electrochem. SOC., Greenwood, I b i d . , 93, 1483 (1908); Pring and Fielding, Ibid., 95, 1501 (1909);D z c f . Chem. IMef. ,Material, 1909, 19; Tucker and Lampen, J . A m . Chem. Soc., 28. 853 (1906).
= LOC. Cit.
