Mar., 1913
1
T I f E J O U R S . 4 L OF I , Y D U S T R I A L A S D E.YGI-\-EERIh7G
ORIGINAL PAPERS
189
I
differences noted are undoubtedly due largely t o the fact t h a t this method shows more than the Thompson B y Vd-. c. SL.4DE. method, in t h a t it is a test for Fligment and vehicle Receixed Dec. 20, 1911. combined. It will be shown later how in certain I n viev of the difficulty in obtaining trustworthy cases the properties of a paint film may obscure t h e results concerning the protective values of paints or property of the pigment (as determined b y the Thomppaint films by tests conducted in the laboratory, son test) contained therein. One of the early tests a n y new laboratory niethod is thereby a p t t o be was the evaporation test.',' An iron dish painted received TT-ith more or less skepticism. It is believed, on the inside is filled with water which is allowed t o ,however, t h a t the method herein proposed is entirely evaporate. The dish is repeatedly refilled after sound in principle; and it has produced results t h a t each evaporation until the paint film has deteriorated, demand a t least thoughtful consideration. I t is or until the time of the test has extended sufficiently. further believed t h a t a little more experimentation From one point of view, the alternate wetting and vi11 make the method suitable for use generally. A drj-ing periods represent the met and dry conditions report of the n-orlc is given a t this time inasmuch in a painted surface exposed in actual service. An as experiments related t o th'e test have been carried acceleration test by Sorton' involves the principle of on a t different times for the past two or three years alternate wetting and drying, b u t is entirely qualiin the Research Laboratory of Applied Chemistry tative in character. The Gardner acceleration test' of the Massachusetts Institute of Technology, and is also a qualitative one and provides for subjecting onl>- the xTery preliminary experiments have been small painted test plates t o artificial atmospheres. published. Recently, a test has been proposed b y Henson and I t is not intended t o do the impossible in proposing Pollock,d n-hich is a sort of electrical insulation test a test t h a t xi11 at once determine the value of a paint carried out in a normal salt solution, wherein the from all points of view or under all conditions. I n film surfaces are subjected t o a potential difference fact, those tests t h a t have been used previously, or of 8 volts. There are a few other tests t h a t have are n o w in general use, are all more or less limited been proposed from time t o time t o determine specific in t h a t they consider only certain ones of the many properties of paint films such as hardness, elasticity, properties desirable in any paint film. A paint for porosity, etc., the majority of which may be found an?- particular purpose should always be selected described in the bulletins of the Paint Manufacturers' because of particular properties i t possesses t h a t Association of the United States. other paints do not possess t o the same degree. That The ideal paint film would be not only uniform and method of testing should be used which considers tough and adhere well t o the surface beneath, b u t most prominently those particular properties. would be at the same time sufficiently elastic t o preUnfortunately, many tests are conducted under vent cracking, and quite impervious t o destructive abnormal conditions, and results thus obtained are gases and liquids. If i t be a case of protecting iron not necessarily identical with those t h a t mould be and steel against corrosion b y means of a paint film, obtained under normal or actual conditions. Thus, imperviousness t o moisture and t o air or to oxygen the Loesner steam test,',' the electrical insulation is the property most essential. This new method determines the degree of imperviousness or the exor voltage testI,2 the heat test ( 2 hours at 400' F . ) , a and tests where films are subjected t o the action of cluding power, and allows all results t o be recorded weak acids or other attacking agents are either un- permanently and t o a certain extent quantitatively, natural in themselres or are performed under un- I t utilizes the principle of alternate wetting and natural conditions. Of course some of these tests drying, and works under conditions made t o imitate are directly applicable and desirable provided the actual conditions as far as possible. paints are t o be used where steam or acid fumes D E V E L O P M E A T O F T H E XIETHOD prevail, or on surfaces subjected t o heat, etc. Some tests eliminate the vehicle, and too often the tenThe method has its origin in the results of experidency is t o predict the behavior of paint films from ments with fruit-can lacquers performed b y Walker the results of experiments carried on with the pig- and Lewis5 in 1909, when i t was shoxn vihy lacquered ments contained in the films, The vehicle is too cans when filled with fruit juices rusted more rapidly important a part of a paint t o neg-lect. The Cushman on the inside than unlacquered cans when filled. knife blade test,' or Perry damp film test,I and the The apparatus used was later utilized t o experiment Thompson testa are all tests performed on the pig- m-ith several paint films. I t consisted of a LT-tube, ment alone. Severtheless, both of these tests have I " in diameter, which contained z o o cc. normal KCl. produced most interesting results with commercial Each arm contained a spiral of soft iron mire, one pigments, and it is t o be noted later hox- closely the containing z j cin of wire, the other I O O cm , and results, particularly of the Thompson test, agree both spirals mere immersed in the electrolyte. The mith the results obtained by this nen- method. The longer spiral bore the dried coat of paint to be tested. A NEW METHOD FOR TESTING PAINT FILMS AND PRESERVATIVE COATINGS FOR IRON AND STEEL.
.
,
CHE-IIISTRY.
= 90
T H E J O U R N A L OF I N D U S T R I A L A N D BiVGIlYEERISG C H E l l l I S T R Y .
The solution around the spiral was kept saturated with air b y constantly bubbling a stream of air through it. The two spirals were connected through a n external circuit in which was placed a bromide coulometer. An electric current developed sooner or later a t the painted spiral, resulting from t h e depolarization of hydrogen ions by the oxygen, a n d the quant i t y of electricity developed was determined in the coulometer. The weight of bromine added per unit time may be considered as a measure of the rate of depolarization. The theory of the process will be considered more in detail later. For later reference, a few measurements obtained are given here. The films t h a t depolarized the least head the column. The thickness of the films was not measured. Coating.
Br. per hour,
w.
Paraffine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc oxide ....................... Graphite ( ...
0.00 0.078 0.078
Zinc chromate.. . . . . . . Barytes. . . . . . . . . . . . . . Graphite. . . . . . . . . . . . .
0.11 1.2 1.87
............
..........
.......... ..........
Carbon black. ...........................
2.4
The work was now taken up b y M. T. Jones who experimented with a number of paint mixtures a n d used the bromide coulometer t o record the various rates of depolarization. The use of the coulometer was later discontinued, however, as its resistance in the circuit during the course of a long run increased greatly, so t h a t the quantity of electricity allowed t o pass continuously fell away from t h a t quantity which would have passed, had the resistance of the coulometer remained constant. For this reason, the results obtained with the coulometer are not given a t this time. Jones next measured electromotive forces b y means of the Poggendorf compensation method, b u t it proved too tedious a n d not sufficiently productive t o warrant its continuance. Direct measurement of current was then attempted with t h e use of a specially constructed milliammeter a n d a few trials produced excellent results. Jones did much towards standardizing the method, and had constructed a n apparatus which was a t first used t o allow for wetting and drying periods when his untimely death occurred. The following year the apparatus was improved and greatly extended, and the investigation continued b y the writer. T H E O R Y O F T H E METHOD.
A brief statement of the theory of t h e method which involves the electrolytic theory of corrosion follows: Consider two identical pieces of iron immersed in water, containing a dissolved neutral electrolyte t o give conductance, a n d connected through a n external circuit. N o current can pass through the external circuit in either direction, for identical conditions of equilibrium obtain a t each metallic surface, and will obtain until this equilibrium is disturbed a t one of the electrodes. By removing hydrogen ions a t the surface of the electrode, as can be done b y depolarizing with a steady stream of oxygen, the equilibrium can be constantly disturbed.
Mar., 1912
At the same time an electric current passes continuously through the circuit. To maintain the equilibrium of ionic charges within the solution, metallic iron must pass into solution a t some point in the form of ferrous ions. This takes place almost wholly a t t h e undepolarized electrode. Of course water is the product of the depolarization, a n d the ferrous ions unite with the hydroxyl ions (which are chemically equivalent t o those hydrogen ions removed) t o form ferrous hydroxide, which in t u r n is oxidized t o the ferric form and t o insoluble rust. A rust coating forms a t the depolarized electrode or cathode a n d so some iron must be going into solution here; b u t i t is only a few per cent. of t h a t moved into solution from the other electrode or anode. The cause for this probably lies almost wholly in local action on the cathode, for practically no depolarization takes place a t the anode. Let the electrodes be in the form of spirals of iron wire a n d the electrolyte consist of normal KCl; a n d consider the cathode spiral to b e covered with a well dried coat of paint and the solution around it t o b e continuously saturated with air. The combination of painted spiral electrolyte saturated with airelectrolyte containing no air will bare iron spiral be called for convenience a “ p a i n t cell.” The paint film may or may not electrically insulate the cathode wire. Probably every paint is porous a t least t o some extent, so t h a t i t is only a question of time as t o how long i t will require for the solution to penetrate the film sufficiently t o establish contact with iron at one or more points. When this occurs;the hydrogen ions t h a t tend t o polarize are depolarized b y the oxygen of the air which is in solution, and the operation of the paint cell commences. Though the driving force of the reaction is not affected b y the presence of the film, t h e rate of reaction or depolarization is greatly affected. The initial current detected is very small-a few thousandths of a milliamperebecause of the great resistance offered b y the film, not only t o the passage of a current b u t t o the diffusion through the pores of the film of the air. With increased porosity comes a n increased rate of diffusion and an increased rate of depolarization and a decrease in film resistance. The current consequently increases. At this time a n y depolarizing power possessed b y linoxyns t h a t may still be unsaturated t o a certain degree should show its influence. The relation between the rate of diffusion through the film a n d the rate of depolarization was well illustrated b y an experiment where several coated spirals, with films slightly disintegrated, were supplied, first with excess air a n d then with excess oxygen gas. I n the second instance the current values more than doubled, returning again t o the original values when air was resubstituted. The relative behavior of films made from different kinds of paints indicates the relative excluding powers t o moisture and oxygen. The increase in current observed in a n y paint cell may be recorded in a current-time curve giving a permanent record of t h e behavior of the film.
+ +
Mar., 1912
T H E JOL-R-VAL OF I S D L - S T R I - A L A S D E S G I S E E R I . Y G C H E X I S T R Y .
191
I t is interesting t o note the analogy between the of a width of 1 3 / ~ “ along the bottom divides the tank arrangement of the paint cell and the conditions longitudinally into two sections. The tanks and existing on a painted surface, injured with scratches partitions are made of galvanized iron well coated or bare spots, of any portion of an exposed steel on the inside with an asphaltum paint. At the back structure. Assuming the injured surface t o be covered of each tank is a series of 14 tubes, closed tightly a t with dew or rain water, then the bare spot or scratch, the top with corks and wax, and extending below the protected or painted surface around the injury .>bout 4” into the KCl solution. Each tube contains and the water covering both correspond respectively one bare iron anode. There are also 14 cathode to the anode, cathode and electrolyte of the experi- spirals which are held in a frame t h a t is free to slide mental paint cell. Of course, in this case the rate u p and down in guides, thus permitting the spirals of depolarization would be less and the resistance t o be removed for periods of drying when desired. t o passage of current through the liquid circuit much The tank is so placed t h a t the cross-sectional dimengreater than in case of the paint cell, but the process sions of any individual paint re11 is as represented
\IEW
O F THE
FIVE UXITSOF
would proceed definitely and steadily, notwithstanding. To allow alternate wetting and drying, t h a t actual weather conditions might be approximated, the apparatus described below was finally adopted. THE APPARATUS.
The apparatus consists of five identical units, each unit having a testing capacity of 14 painted spirals. A view of the five units is given in one of the accompanying photographs. Each unit includes a n electrolyte tank measuring approximately 2 S 1 / 2 ” X 63/,” Y SI/,”, filled t o a depth of 7 ” with normal KC1 solution. A metal partition with a cloth diaphragm
THE
PAINT-TESTING APPARATUS
in Fig. I . A photograph of an assembled unit with the tank removed is also given. The air is supplied by tubes held in position b y a frame which can be so adjusted t h a t each tube can deliver a stream of air bubbles directly up through a cathode spiral. This insures the continuous saturation of the solution in the immediate vicinity of the electrode. On the other hand, the anodes are so placed and so shielded from access of air (except t h a t small amount which may diffuse down through the solution and up inside the tubes) t h a t the oxygen content within the anode tubes is practically depleted all the time. Each cathode is connected t o its corresponding
Iq?
T I I E J U L I I S . 11, O I ; I X l j US'TlU.1 I. . I .\-I)
anode through an external circuit. The connections are all made in a switch-box, shown in thc photographs. in such a manner t h a t when the paint cells are working, each cell scnds its current through an individual roo-ohm resistance coil; and when mensnrements are t o bc made, anii a milliamnieter with an intcrnal resistance of I O O ohms has been connectcd to the binding posts of the boa, then, b y moving a radial contact arm, the current irom any individual paint cell may bc s c n t at once through t h e roo-ohm coil of the rriilliammeter instead of through its rooohm coil in the svitch-box. Each paint cell is entirely distinct from its neighbiirs. The fourteen paint cclls have oiily t h e elcctrolytc in common. The use of thc rcsistancc coils makes it possible to read
xzll-,, , < ) I ?
contacts at dcsired intrrvals such t h a t electric circuits arc m:idc. X r l a y is thereby hrought into play and a larger current passes t,hrough a motor ( X j which controls the raising ani1 loxrering rnechanisnl
Accorrling t o t h e imrticular contact t h a t thc h o u r hand makes, tho frames arc raised or lowere& For tests recorded in this paper, the wiring wiis arranged t o give lifting contacts at j , 6, 9 and 1 2 dclock a n d h v c r i n g contacts a t I : 4, 7 and I O o'clock. Thcrp n w t i t h u s 8 onr-hour dryin?: periods a n d X two-hour wciting pcriods per 24 hours. >ir:ils are rnadc. of a soft iron wire, X- the selection of a perfectly arbitrary point, b u t a selection t h a t might prove misleading, another method has been adopted. The curves are current-time curves, and hence the area included b e t r e e n any curve and the co-ordinates corresponding t o any point on the curve represents a quantity of electricity. The magnitude of the included area or of the quantity of electricityin coulombs, for example-is directly proportional to the amount of depolarization t h a t has proceeded up t o t h e same time. The point of time t o which t o integrate along any curve has been determined
60
SO
4c
30
2C
IO
0
20
40
60
be t h a t , though the resistance t o the passage of the current does not decrease very rapidly, the resistance t o the diffusion o f air does decrease sufficiently rapidly t o cause the rate of depolarization t o decrease very fast, especially after the most pronounced maxima. The choking of the pores of the film bS; even a small amount of rust 1%-ouldaccount for this T H E E\-ILU.'.TIOS
O F P R O T E C T I V E 1-ALUES.
I t would seem most natural t o refer all curves t o t h e bare iron curve, calling some value along the curve t h e value for zero protection But t h e curi-e covers such a range of current values t h a t it is difficult t o select any b u t perfectly arbitrary points. I t does not seem justifiable t o select o 466 mg. for the value, as, when this is reached, considerably more rust has
IO0
I20
140
160
180
200
in several ways. and thus far no one way has been adopted in preference t o the others. The different methods lead t o essentially the same results. Thus all areas up t o (,4) 7 0 hours or i B ) L O O hours of wetting might be integrated; or C'C) integrate t o t h e points where t h e curves pass through some ordinate, as at 0 . 2 0 milliampere. Again, the maxima of the curves might be used as points t o which t o integrate. I n this case, t h e time a t which the maxim u m occurs must be considered. Let i t be assumed t h a t t h e sooner the maximum occurs t h e more rapid t h e deterioration of t h e films; and t h e greater the magnitude of t h e maximum, t h e greater t h e depolarizing power of t h e film which, in turn, depends directly upon decrease in excluding power or increase in deterioration. I n other words deterioration may
T H E JOL-R.YA4L OF I L Y D U . 5 T R I A L A.YD E S G I S E E R I S G C H E i I I I S T R Y .
198
be represented as directly proportional t o t h e area integrated and inversely proportional t o the time or t o some power of the time. Protective power may be represented as the inverse of this. Hence, P = C t" / A = K t * / Q , where K is a proportionality constant, t the time u p t o the maximum current, Q coulombs passed through t h e circuit up t o the time t , a n d 12 an arbitrarily chosen constant. Relative values have been obtained by calling K unity and b y assigning to the exponent n the values ( D ) I a n d ( E ) 2 . This method may be used only in case of the one-coat tests. The quantitative results of the above methods are, for the sake of brevity, summarized below in a qualitative manner. The capital letters refer to the preceding methods, and the fikures t o the paint mixtures as follows: ( I ) zinc oxide, ( 2 ) zinclead chromate, (3) American vermilion, (4) red lead, (j) white lead, (6) iron oxide, ( 7 ) barytes, ( 8 ) silica, (9) carbon black, ( I O ) graphite. The higher the position of a number in any column, the greater the excluding power, and as a rule, the less the depolarizing power of the paint film. One-coat tests.
TKO-coat tests. A,B.
C.
1
-
2 10 9 4 3 5
7 8 6
-
A.
B.
D.
E.
1
1
2
2 9
2 1 10
2 1 9 10 5 4
10 9 4 3 5 7
9 10 3
8 6
5
4 8 6 7
10 3 8
9
4
3 4 8
6 7 5
6 5 i
3 6 8 I
WORK NOT REPORTED
Although the method uses current values, a large number of electromotive forces were measured, taken after the current readings and after breaking the circuit. Data relative t o the change in resistance of the film were thus obtained. Direct measurement of resistance is greatly disturbed b y the condenser-like action of the paint films. All this will probably be considered more fully later. An attempt was also made t o determine the cause for certain inexplicable deviations among the average values of the current readings t h a t sometimes occurred during the course of a test. An examination of Plots I a n d I1 shows a few widely displaced points, especially on the third curve for red lead. All conditions were guarded as carefully as possible : the electrolyte was kept a t constant level, thus preventing change in conductance due to change in concentration. Temperature changes were watched. The resistance of the electrolyte between bare iron cathodes a n d anodes was only 18.6 ohms, which formed a p a r t of the resistance of the total circuit of about 119 ohms. This being the case and, further, the temperature changes in the tanks being small, no corrections were made in the current readings. These corrections would have accounted for b u t a few per cent. of the wide deviations referred t o above. Except for a few days when the temperature varied 18 5 4 O , i t varied only 19 5 2 " . The change in solubility of
Mar., 1 9 1 2
oxygen was thus very small, and could not account for the deviations anyway, as they were frequently in the wrong direction. c o x c L us I O h-
The work done thus far indicates t h a t the method is sound in principle, reasonably practicable, and reliable, in t h a t under the same conditions of testing, results can be duplicated. I t determines definitely, a n d under artificial conditions made as nearly natural as possible, the excluding power of a n y paint film against water and air. As regards the testing of paints for use on iron and steel, the method should develop into one of especial value, as the nature of the corrosion through the painted surfaces (according t o the electrolytic theory of corrosion) is entirely analogous to the mechanism of the reactions in the paint cell. I t has further been shown t h a t , a t least under the conditions maintained in the test-and it is probably true to varying degrees under conditions of actual service-films prepared from certain of the pigments experimented with possess a marked depolarizing action. This may be in part the saturation of unsaturated bonds in the linoxyn b y addition of nascent hydrogen, as i t has been shown t h a t linoxyn will depolarize hydrogen under the conditions maintained; and i t may be due in part to depolarizing properties possessed b y the pigments themselves ; b u t i t appears t h a t the rate of depolarization is determined largely b y the character of the whole filmt h a t is, pigment and vehicle combined. Increasing the porosity or the thinness of a film makes it possible for the depolarizing action t o appear, b u t it apparently does not necessarily cause i t t o appear. The exact nature of the depolarizing action, and its true relation to the thickness of the film will be investigated more fully in the future. REFEREXCES. 1 Gardner and Cushman. "The Corrosion and Preservation of Iron and Steel," pp. 241-243. 2 Gill and Foster, Tech. Quarterly. 17, 145 (1904). 3 Thompson, Trans. A m SOC. T e s t i n g iMalerzals, 7, 493 (1907). Benson and Pollork, THISJOURNAL, 3, 9 (1911). 5 IValker and Lewis. I b i d . , 1, 11 (1909).
RESEARCHLABORATORY O F h P P L l E V CHEMISTRY, > f A S S . IXST. O F TECHNOLOGY. BOSTON.
A SHORT METHOD FOR THE DETERMINATION OF SOLUBLE ARSENIC I N COMMERCIAL LEAD ARSENATES. BY B. E. CURRYAND T. 0. SMITH. Received October 23. 1911.
As the result of a request for an immediate report on the amount of soluble free arsenic oxid in a sample of commercial lead arsenate, the writers began a series of experiments t o develop a method of analysis which would give satisfactory results without incurring the tedious shaking of two liter bottles, eight times daily for ten days as in the procedure outlined in the standard A. 0. A. C. method for this determination. Since i t was desired mainly to reduce the time,
