INDUSTRIAL A N D ENGINEERING CHEMISTRY
1084
Yol. 23. S o . 10
Prevention of Corrosion of Metals by Sodium Dichromate as Affected by Salt Concentrations and Temperature' B. E. Roetheli and G. L. Cox RESEARCH LABORATORY OF APPLIES CHEMISTRY, DEPARTMEZIT OF CHEMICAL ESGINEERISG, ~ZASSACHUSETTS CAYBRIDGE, MASS.
The effects of additions of soluble chromates to distilled water and sodium chloride solutions, a t room temperatures, on the corrosion rates of steel, wrought iron, zinc, galvanized steel, copper, brass, aluminum, and lead were investigated. Additional studies were also made in the cases of steel, wrought iron, and zinc of the effects of increasing the temperature on corrosion rates in similar solutions. Chromates were found suitable for the prevention of corrosion of wrought iron and steel a t all the temperatures investigated in distilled water, 0.003 per cent, 0.05 per cent, 3.5 per cent, and 22 per cent sodium chloride solutions. Aluminum was almost completely protected in presence of chromates a t room temperatures in dilute salt solutions and corroded slightly but locally in the strong salt solutions. Partial protection of copper, brass, galvanized steel, lead, and zinc from corrosion in distilled water and .
R
APID developments in recent years in the efforts t o prevent or retard the deterioration of metals in water
and salt solutions have led to the use of certain substances known as passivifying agents, such as chromates and dichromates, in the corroding solution. By the use of these substances the rate of deterioration of metals in water and aqueous salt solutions has frequently been reduced to small or negligible values. It has long been known that if iron or steel be subjected to the action of a strongly oxidizing agent, such as chromic acid or soluble chromic salts, the metal is rendered passive, and the mechanism has been attributed to the deposition of a thin oxide film impervious to further attack (3, 5 , 6 , 8, 9, 11, 12). Other authors (1, 2, r, 9, 10, I S , 16, 19, 20, 21, 23) indicate that chroniates retard the corrosion of iron even in the presence of salts such as chlorides and sulfates, and somewhat in dilute acids. However, it has been demonstrated that the presence of chloride ions in solution tends to decrease the protection obtained (11, 12). Various methods have been devised (16, 27) whereby the metal is protected by an artificially applied coating of a cement containing the chromate. bIost of the above observations were made on iron, steel, and associated metals, such as chromium and nickel, and little is known regarding the effect of these highly oxidizing agents on other common metals, such as zinc, copper, aluminum, and lead. However, it is known that aluminum is rendered passive in the presence of chromates, and its corrosion rate retarded appreciably (4, %). Another author (83) claims that the dissolution of zinc from galvanized iron is accelerated in the presence of chromates, and that the attack is decidedly isolated. Still other investigators (18, 22, 24, 26) claim that galvanized iron is appreciably protected when chromates are used in strong brines. Although the retarding effect of chromates on corrosion was discovered as early as 1907, it n-as not until quite recent 1
Received June 6, 1931
INSTITUTE OF T E C H X O L O G Y ,
salt solutions a t room temperatures was obtained by the addition of chromates to the corroding medium. The necessity for the addition of sufficient quantities of chromate is emphasized by the results, in t h a t wrought iron, steel, zinc, and aluminum were severely pitted in the salt solutions when chromate concentrations were too low. Increasing the salt concentrations, in general, progressively accelerated the corrosion of the nonferrous materials, whereas wrought iron and steel corroded more rapidly in dilute solutions t h a n in distilled water or cqacentrated solutions. Increasing either the temperatures or the salt concentrations made necessary the use of greater quantities of chromate for the same degree of protection obtained a t lower temperatures or in the more dilute salt solutions. The differences in degrees of protection obtained are attributed to differences in the nature of the film formed on the metal by reaction with the retarder.
years that tlie principle was given practical application. The real benefit of the retarding action of chromates on corrosion rates was first derived in the treatment of brines in refrigerating industries for the prevention of the corrosion of ice cans and cooling systems. Since 1925, many recommendations have been made (9,14,17,18,22,Z4,26) regarding the optimum amount of chromate and the degree of alkalinity of the solution necessary to afford the best protection. Most of these investigators agree that the brine should be neutral or slightly alkaline for good results but differ in their opinion as to the optimum concentration of the chromate. The recommended values vary from 0.08 t o 3.6 grams of sodium dichromate (neutralized to a pH of 7 to 7.5 so as to obtain the salt in the chromate form) per liter of brine. It has been observed that insufficient quantities of the chromate result in isolated attack with subsequent severe pitting (9, I O ) , hence laboratory investigations are essential for the determination of the necessary concentration of the chromate to prevent this isolation of the attack. As previously mentioned, little information is available regarding the retarding effect of chromates on the corrosion of the common metals and alloys other than iron and steel in Ivater and salt solutions. Furthermore, the effect of temperature changes has not been considered in detail. Therefore, for the purpose of obtaining information regarding the limitations of the use of chromates as retarders for a number of metals of commercial importance, an investigation was undertaken whereby the effects of salt concentrations and temperatures on their protective influence could be studied. Determinations of Corrosion Rate
EXPERIMENTAL LIETHoD-The method employed in this investigation was designed to permit the gravimetric determinations of the corrosion rate a t different temperatures of a number of metals in sodium chloride solutions containing
INDUSTRIAL A N D ENGINEERING CHEMISTRY
October, 1931
varying quantities of sodium chromate.2 The design of the apparatus permitted maintenance of constant temperature, constant concentration of the solutions, continuous aeration, and definite velocity of the specimen through the liquid Sketches of the equipment used in this investigation for determining corrosion rates are shown in Figures 1 and 2. I
Figure 1 shows the apparatus used for the corrosion-rate determinations a t 20" to 24" C. The corroding solution was contained in the 4-gallon earthenware jar, fitted with a level gage, and the water evaporated from the solution was replenished by additions of freshly distilled water when necessary. The solution was continually aerated by introducing air through a glass tube extending to the bottom of the jar. The specimens were suspended on glass rods firmly attached to a wooden disk and rotated by means of a system of shafts and gears operated
I
WOODEN DISC
Figure 1-Rotating Tester f o r RoomTemperature Runs
from a reducing gear and motor. The speed of rotation was maintained constant a t approximately 56 r. p. m., corresponding to linear velocity of the specimen through the liquid of about 36.6 centimeters per second. The relative velocity of the specimens through the corroding medium was determined by measuring the speed of rotation of the disk and speed rotation of the liquid in the same direction. The linear velocity was computed a t the mean diameter of the plane cut by the rotating specimen. The apparatus for the elevated-temperature determinations, sketched in Figure 2, was operated in a similar manner. A heavy Pyrex glass jar with a capacity of 15 liters was used to contain the corroding solution. The jar was immersed in an oil bath which was heated by an immersion-type electric heater, controlled externally by a variable resistance. When the external resistance was adjusted for any particular temperature, the temperature within the jar was maintained substantially constant. The cover for the jar was made from a heavy steel plate, coated on the inside with a corrosion- and temperatureresistant paint, and fitted to the jar by means of a rubber gasket. Screw clamps were employed to hold the plate in position. The cover of the jar was constructed to provide for the insertion OF a thermometer, an inlet tube for aeration, a level gage, and a water-covered condenser to minimize losses by eyaporation. Water lost by evaporation was replaced by additions ,of distilled water. The variations in the concentration of the corroding media never exceeded 5 per cent of the initial values.
1085
were cut from sheets of the metal of thicknesses of 0.06 to 0.19 em., each specimen having approximately the same total area as the iron and steel pieces. All specimens were drilled with two 0.635-centimeter (0.25-inch) holes near the ends to facilitate suspension in the liquid. Before immersion the specimens were treated with 10 per cent hydrochloric acid to remove the adhering mill scale, scrubbed with steel wool and soap and water, washed with fresh water, rinsed in reagent alcohol, dried, and weighed. Upon removal of the specimens from the corroding medium, the corrosion products were removed with a fiber brush, scrubbed lightly with steel wool, and again washed, rinsed, dried, and weighed. For all the metals, except steel and wrought iron (in which cases it was negligible), a polishing correction was determined to insure more accurate results. On metals, such as zinc, brass, copper, aluminum, and lead, the abrasive action of fine steel wool was appreciable. Hence, after the second weighing, the specimens were scrubbed in exactly the same manner, and this additional loss in weight was deducted from the gross loss in weight to give the net loss by corrosion. This correction was usually small as compared d h the net loss. Throughout the entire investigation, the types of corrosion, the appearance of the corrosion-products film, and the extent of the pitting were noted. Photographs were made of some of the typical cases of isolated attack, and maximum penetrations were determined from the depths of the pits in cases where the pitting was severe. The depths of the smaller pits were determined by means of a calibrated microscope by focusing on the bottom and top edge of the pit. The accuracy in the determinations of the depth of pitting is estimated to be within about 10 per cent for large pits and 15 per cent for the small pin-hole type of pits. The maximum temperature variation for the room-temperature determinations was * 1O C., and for the higher temperatures, *3O C. The normal deviations from day to day were usually not quite so large. Temperature readings were noted twice daily to obtain an average value for the duration of the runs.
"0
The corroding solutions were made from distilled water and sodium chloride. Sodium chloride was selected as the salt because this substance has been shown to reduce the effectiveness of a chromate most markedly in retarding corrosion. The concentrations, 0.002, 0.05, 3.5, and 22 per cent by weight were used to correspond, .approximately, to the average total salt contents of soft wat'ers, average industrial waters, sea water, and refrigerating brines, respectively. The specimens for the tests on steel and wrought iron were cut from 6.1-meter (20-foot) lengths of 2.54-centimeter (1inch) commercial pipe, each specimen being 10.15 centimeters (4 inches) in length, with a total exposed area of approximately 190 sq. em. The specimens of the other metals 3 Throughout this paper, although the figures indicate grams of Nad2rzOrQHzO added, the retarder will be designated as chromatic since the solutions were neutralized to convert the dichromate to chromate.
IRON BOX
Figure 2-Rotating Tester for HighTemperature Runs
REbrLTb--The results of the investigation are given in Tables I to VI and Figures 3 to 8. In the tables are shown the corrosion rates of a number of commercial metals, including steel, wrought iron, zinc, galvanized iron, brass, copper, aluminum, and lead, expressed as penetration in centimeters per year in the various solutions a t the different temperatures. Each value of the corrosion rate represents the average penetration value of two check specimens based upon the losses in weight of the metal, with the exception of the data on zinc in Table IV, in which the maximum corrosion rates are based upon the depth of the deepest pit on either specimen. The average deviations from the mean losses in weight of the
check specimens were * 7 per wnt for the specirneris corroded in solntions containing no elmmate, aiid * 15 per cent. in chroniat,esolutions. E'igurus 3 to 8 show pIii,tiigraptis of sonie of tlie t.ypica1 rases oE isolatcd corrosion and pitting. Corrosion of Steel and Wrought Iron
Since tlierc appears to lie no appreciable difference in tlie behavior of steel and wronght iron under tlie conditions of testing, these two metals will be consideri:d nnder inie heading. In Tables I and I1 arc shown the cormsbn rates of these inetals based n p i i the average losses in weight of the specimens over 5 14-day period, in tho various solations at the different bemperatnres.
inilte are neaeastiry to forrii and miriirtain filins wliich are homogencms and protcctive.3 If insiificicnt. qnantities of e l m inate are present, films eanniit. be rebuilt a t weak p i n t s , and corrosion nray prirceed \with the protected areas acting cathodically so that large areas itre available for depolarization, and the metal will dissoln? at liigir ratcs at very small anodic areas.
Tahle I-. .Currolion of Sfeel (Bared upon loss in weight ai spe~itnensover l e d o x period 01 I 4 days) NaCL "Y -. . *rxa*(i,s 1~aNuinaTl"W..w ~ ~ ~ ~o y , 0.1~ T O..P~ ~ I.V- ~ .m n 4.01
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96
O.ORRS 0.001s o.onn1 n.uavi O.IMOI n . n m 0.1io6 n.ni?ob 0,0004 o.oono 0 . 0 ~ 00 . 0 ~ 0 0.0529 0.0441b 0 . 0 1 3 l b 0.0007 O . M u 0 0.wOO
Corrosion of Zinc
111 aiid I V are sliowrr tlit: nlrrosiorr rates of zinc lased upon the average kiss in weight tlic specimens and the depths of the deepest pits, respectively. Apparently as regards zinc, nierely considering tla: average penetration of the metal by no means affrirds an ncenrat.e niea~ureof the extent to whiali tile niet,nl has deteriorated. In some o w s where tlie average rate of corrosinn indicates an apparent protection by the chriiinatc, tlie specimen failed a t certain isolated areas (see Tables 111 arid l r , spceiinens corroded in 22 per cent salt siilution at, 1.5" ('. containing 2 grams of the neirtralized dichromate per liter). IIence, any discusriiori uf this particnlar metal must deal with the niaxinunn corrosive effect as indicated in maxiniinn r&s of jxnetrrttion at pitted arcas. The data of Talde I\' indicate tirat the retarding effect upon the corrosion rate of zinc in salt solutions is not to he definitely deterniinrd. The nrrtal apliears t o be safely protected in dilute salt solntions and at low temperatures (20' C.)* hut as the teinperatures are iiicre 95" C. the rate id iicti;rior:itirrn iiE tlia nietal increases considerably, and no cflectin: pr&ertim is obtained by ad& tions or tiie ciir,jmate. 111 ' l ' a l h
In gcuerd, at aiiy definite ei,iioentratiiiri OS halt and t e n peratitre, the arlditioii of elirinnat,c to the s i i l u t i o n results in an appreciahle decxase ill the ovcr-all corrosion rate. Increasing the salt concentration or tbe temperature results in increasing the miniinurn connent,ratiiui of retarder necessary for jiroteotion of the niet,al. :is siispceted, insufficient conr:entrat,iuns uf tlie clironiattr tmd t,o ORIISO isolated attack rind conwqucntly pitting of varyiiig ilr~grecs of severity. In Figure 3 are showti t1,e types OS isrrlatcd iLttiic1i diserved in the caac of tliese metals whni tho chrornatc i:mcentration vas too lim for complete Irotcetion. Tlio specimens siiown in the photograph were iif steel whidi w a s cnpoied a t 75" C. to a 0.05 per mnt sodirno chloridc sijlntirii~contrtining 0.1 gram of neutrnlioed sodiuni diclirwiatc per litcr. 13irtIi tlie spot. and atrcak type of isrilated corrosion \vert? oiiaer*etl to occur on both st,eel and rvrought iron in tire various salt solntions and scemed to occur indiscriruinately if t,lie qnaritity of tile retiirdor nsed ~ a . 3insuficieilt for complete probeotion. Apparently, certain ininininni connclit~rations of el~ro-
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In Yigures 4 to 0 are shon-rr piioii,graplw (after rcmovd d t h e corrosion products) uf the siiriaces of speciinens whicli liad bceii corroded a t the differelit tempemtores. It is evident that the metal fails milch more rapidly i i i distillt4 water or salt soliitims at 75" or 95" C. than at 20" C It ir also apparent that the presence of chrrrnrate teiids to ciiiist' isolation of tile attack a t small areas, resnltiiig in a failurt! of tlie metal even wlicii the average loss in weight of tile speciwen pcr unit itreit is small. The pliotographs clearly indicate that an iocrcilsc in t,he salt concentration or iii the teniprature of tlre solution increases the minimum quantity $81 tlre chromate necessary to afford any appreciable protectirm A IJeculiar condition was noted in the ease of the corrosi(iii of zinc in 3.5 per cent salt sointion a t 75" and 85"(:., the soiiltions cniitainirrg 3.0 grarns of the dicliroiriate per liter. A dense yellow coat.ing, difficult to relnove, was deposited o i l the metal surface a d protected tlie nietal from further attack, and tlie corrosion was not isolated. (This filiri was partly removed from the specinlens hefore the pliotograph WRI.
P I' 'I
0.330 0.822 11.3Y
P
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(1.410 P I'ded P
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made.) Appareiitly the prcseiiee vf chlurides iii tliia w i i (:i!ntration did not permit of tlie fiirmation of ark oxide film so t,Iiat an almost cvmpletely unifimn anodic produet of ziix i.lrn,niate was formed which adeyiiately protwted the metal Srim t,he aeiion of oxygen. In 22 per cent salt solution, a t approsiriiately tho salne colltent of ehrimiaic, thc specimen failed a t
