Alternate Immersion Corrosion Test Equipment IMPROVEMENTS IN DESIGN CHUK-CHING MA School of Chemical Engineering, Tulane University, New Orleans, La.
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framework. When the experiment is in process, the bath is maintained at a fixed temperature by means of an immersion heater which is controlled by a temperature regulator. This is of the sensitive liquid-mercury column instrument type. A relay turns the heater on and off, so that the temperature is controlled t o within 0.5' C. I n the temperature range of 30' to 40" C., this variation' occurs along a vertical traverse of the bath. At any given depth, the temperature is the same throughout the bath. Water may be circulated if so desired. This water bath is put inside an insulating hood, so t h a t the humidity in the chamber can be controlled to the point that the specimens remain completely wet throughout the time t h a t they are out of the solutions. The driving mechanism consists of a fractional horsepower electric motor, which is geared down t o 8 r.p.m. through a selfcontained gear box. The motor is connected t o a 48 to 1 ratio gear reduction unit by a set of 1 to 2 ratio gears, thus turning the input of the reduction unit at 24 r.p.m. This gives a n output of one revolution every 3 minutes. The gearing ratio may be changed to prolong or shorten the time for one revolution to meet the specific requirements. Power is taken from a pin and free-turning sleeve arrangement attached t o the face of the output disk of the gear reduction unit, 2 inches from the center. A length of waxed cotton twine is tied DESCRIPTION O F EQUIPMENT to the sleeve and passed over a pulley. Attached t o the other end of the twine is a sample-suspending The alternate immersion corrosion test equipment was conplatform. This is fabricated from 1/4-inch heavy-walled glass tubstructed in compliance with the tentative method as specified ing. Two main rods, 32 inches long, support seven shorter rods, by A.S.T.M. Designation B 192-441' (I), with a number of 21 inches in length. These short lengths are spaced about 5 inches modifications. The first characteristic feature of this new design apart, and are tied with wire at right angles t o the two longer rods. This construction at once affords lightness, rigidity, and is the hanging of the test samples in a position t h a t will ensure adequate strength for laboratory testing. Two supporting equal durations of immersion of all parts of the samples in the lengths of waxed twine tie t o opposite diagonal corners. A seccorrosive media. Another feature is the continuous up and down ond set of these cords is added t o ensure against mishap caused by movement of all the samples under test, so as to distribute uniattack by vapor from the corrosive agents. Thus, as the disk and the formly the corrosive condiattached pin turn when the tions over each one of the motor is in operation, the specimens and to prevent tendmain cord alternately raises ency for strongly localized and lowers the sample-bearing platform in a cycle of 3 attack due to the presence of minutes-1.5 minutes descendc o n c e n t r a t i o n c e l l s . The ing and 1.5 minutes ascendthird feature is its structural ing. The total excursion of simplicity and the convenience the platform is 4 inches. This is adjusted by the relative of conducting the corrosion distance of the pin from the test. A photograph of the center of the disk. Hung equipment is shown in Figure from the short rods of the 1. Another photograph has supporting platform by means of glass sewing thread, the been published previously ( 2 ) . metal samples are alternately The size of the water bath, raised out of and lowered into the number of specimens for a number of corrodant-cona single test, the time cycle, taining beakers (or glass jars) etc., may be varied from case to placed in the water bath below. The level of the solucase. tion in each beaker is the same as the level of the water For conducting experiments in the bath. The loss of with this equipment, a water water i n the water bath or bath 3 feet long by 2 feet wide in the test solution by evapoby 6 inches deep is made of ration may be offset bv the l/az-inch galvanized sheet iron. addition of water or corrosive Figure 1. Photograph of Alternate Immersion The four sides and the bottom Corrosion Test Equipment medium. The spaces between are reinforced by a wooden
HE usefulness of a metal or an alloy depends primarily upon its mechanical strength and its corrosion and erosion resistances. In most cases happening in chemical industries, the failure of ferrous or nonferrous structures are due t o corrosion rather than mechanical wear. Before a material is selected for construction purposes, therefore, it is important to make a thorough corrosion study with regard to its suitability to that particular service. Corrosion resistance of various metals and alloys or of one material with different surface conditions may be compared by testing in the laboratory or in actual service. The alternate immersion corrosion test has long been an important method of conducting laboratory corrosion experiments. The present paper has been written largely on the improvements in designing laboratory equipment for the alternate immersion corrosion test. The more important variables t h a t must be under control in a corrosion process are well considered in the new design.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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the neighboring beakers are large enough to permit free circulation of the water around the beakers. EXPERIMENTAL ARRANGEMENT
I n all corrosion testing, it is best to prepare specimens from strip metal so that the ratio of the surface to mass is large and the ratio of edge area to total area is small. All the specimens used in the present investigation are cut to the size of approximately 1 X l / z X 0.05 inch. Care should be taken to ensure parallel sides. They are finished until free from rough surfaces, sharp edges, and contamination of the metal surfaces by grease or other undesirable foreign matter. The shape and dimensions
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FINNULAR &LASS BEADS
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Vol. 44, No. 3
The paraffin is found to be quite necessary because the glass thread, for all of its great tensile strength, is very easily cut by the slightest rubbing against the relatively sharp edges of the sample holes. The 1inch of paraffined thread is found to be satisfactory in resisting cutting action. Also, unless paraffined, the knots, although tightened, have a tendency to undo themselves. The assembled groups are then hung on the short rods of the supporting platform in their respective horizontal positions. The depth of each group below the glass rod is adjusted to a predetermined distance, which is based upon the liquid level in the beaker. In case of any misadjustment of the 50% in and out timing in the alternating immersion cycle, it can be readjusted by raising or lowering the whole platform plus specimens by changing the effective length of the main supporting string from the pin and sleeve on the output disk of the gear reduction mechanism to the supporting cords of the platform. Constant liquid levels are maintained in the beakers by the addition of fresh corrosive solution or distilled water, as the case may be. In no event does the concentration of the corrosive agent change by more than 8%. A sketch of a group of two specimens hung from a short rod of the supporting platform is shown in Figure 2.
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EXPERIMENTAL DATA
In order to study the reproducibility of results obtained from experiments undertaken with this equipment, corrosion rates of anodized and unanodized titanium in sulfuric and hydrochloric acids were summarized in Tables I and 11. Triplicate runs were performed for each acid concentration and the percentage deviation of each individual test from the average value of the three was calculated and included in the tables.
Figure 2. Sketch of Two Samples Hung from Supporting Platform DISCUSSION AND CONCLUSIONS
The main objects of laboratory corrosion tests may either be to of specimens thus prepared will permit weighing on an analytical balance and facilitate accurate measurement and calculation of seek the gaining of pure scientific knowledge of the mechanism of the corrosion process undertaken or to find a suitable material the exact area of each sample for corrosion testing. These requirements meet the standards as recommended by the Amerito withstand a particular service. In finding a suitable material to withstand some particular service, it is highly important to can Society for Testing Irlaterials ( 1 ) . Two small holes are recognize and subsequently control the variables that are likely drilled through the face of each specimen on its major axis, '/a to affect the desired result. Whether these ponderous variables inch from each short edge, t o accommodate the supporting thread. are factors associated with the specimen itself, such as surface In the comparative study of corrosion resistance of anodized condition or variations in composition of the sample, or are conand unanodized titanium as conducted in the author's laboratory, nected with the environment, such as degree of oxygenation or the specimens are divided into a number of groups, each conconcentration of the corrosive medium, it is necessary to study sisting of one anodized and one unanodized sample. Only one them thoroughly. In combination with a knowledge of the group is suspended in each beaker. A4sthe two samples are composed of the same metal and taken from the same metal strip, the corrosion products will not affectthe results to a noticeable extent. TABLE I. CORROsION RATESO F A4NODIZEUT~TASIUN CONDUCTED AT 35" C . Each group of specimens is strung together A I i D A DURATION O F 120 HOURS with a 15-inch length of glass sewing thread. corrosive conon,, Corrosion Rate, hfg./Sq.Dm./Day Deviation from Average, 7% The unanodized sample of each group is insulated Medium Wt. % I I1 I11 Av. I I1 I T electrically from its anodized companion by two Sulfuric 10 0.54 0.56 0.60 0.57 5.26 1.75 5.26 1.10 1.04 1.06 1.07 2 . 8 0 acid 2.80 0.94 annular glass beads. 1.97 The stringing procedure is carried out by first making a small untightened knot in one end of the glass thread. This knot, together with about 1 inch of the thread length is dipped into molten paraffin, removed, and allowed to cool. Next, the other end of the thread is passed through one of the two holes in the sample at the bottom and thence through the hole in the glass bead. The thread end is next passed through one of the holesin the sample at the top and back through the remaining hole in this sample. The thread is then passed through another bead and finally through the remaining hole in the bottom sample. Another untightened knot is made and, along with another 1-inch length of the thread, is paraffined and allowed to cool.
Hydrochloric acid
40 60 10 15 30
1.95 2.18 2.03 97.4 99 4 98.20 102.7 0.88 0.92 0.93 0.95 '221 215 208 217 706 687 683 672
3.94 1.21 1.09 3.26 0.58
2 96 3.32 3.26 0.93 2.18
7.38 2.01 4.37 2.80 2.76
TABLE 11. CORROSIOE; RATESOF UNANODIZED T~TANIUM CONDUCTED AT 35' C. AND A DURATION OF 120 HOURS corrosive Medium Sulfuric acid Hydrochloric acid
conon,, Corrosion Rate, ,lfg./Sq.Dm./Day Deviation from Average, % Wt. % I I1 I11 Av. I I1 I11 2.53 6.75 4.14 22.7 23.1 23.7 10 25.3 0.70 5.92 6.62 306 287 270 25 285 0.98 2.09 732 717 1.26 708 710 40 3.31 7.21 60 98.6 99.2 110 102.6 3.90 0.59 2.38 1.69 1.72 1.68 2.98 10 1.63 244 2.46 3.70 0.82 253 242 15 238 3.07 0.61 3.53 674 631 651 30 647
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INDUSTRIAL AND ENGINEERING CHEMISTRY
complex nature of the corrogion processes and the numerous variations in environmental conditions, reliable results can be obtained only through the skillful adjustment of the various internal and external factors. The alternate immersion corrosion test has its chief advantage in allowing the investigator to set these variables in a relatively simple manner at fixed values, while varying those deemed to be important to the test. For example, with this equipment, oxygenation can be accomplished while the specimen is out of the corrosive medium but still retaining a liquid film on its surface. The velocity of the liquid past the specimen is controlled automatically by the duration of the cycle and the distance of descent of the sample into the solution. The continuous up and down movement of the specimen within or without the solution will avoid the uneven distribution of the corrosive conditions over the specimen. This will minimize localized attack which may not exist in actual service conditions. All parts of the specimen will be immersed almost simultaneously into the corrosive medium during the descending cycle, because the specimen is hanging from the platform in a horizontal position upon its thin edge. Based on the experimental data as listed in Tables I and 11, the reproducibility of results obtained from corrosion test carried out in this new equipment is in the range 1 to 7%. Inasmuch as a 10% accuracy is sufficient in this type of experiment, it appears
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that the results from these tests can be used as a basis for selecting material for construction purposes. The difficult variables are apparently controlled to a point that can match closely the actual conditions in service of the materials that are being considered, Furthermore, this apparatus may be employed advantageously for studying effects of variations in composition or heat treatment of a material. Besides experiments on titanium, corrosion tests on aluminum and stainless steel have been carried out. The results in these tests can be reproduced within the limit of about 5%. For more details about corrosion resistance of titanium, previous publication on this subject (9)should be consulted. Finally, the glass sewing thread used for hanging specimens is found t o be satisfactory to withstand the attack by acids, salts, and alkali solutions of medium strength. This thread consists of three strands containing 300 filaments per strand. It is strong enough to withstand a 60-pound pull. LITERATURE CITED
(1) “A.S.T.M. Standards,” Part I1 (B 192-44T), pp. 804-16, Philadelphia, American Society for Testing Materials, 1949. (2) Ma, C. C., and Peres, E. M., Jr., IND. ENQ.CHEM.,43, 675-9
(1951)
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RECEIVED for review May 19, 1951.
ACCEPTED August 22, 1951
Constant Liquid Feed Device A. H. MAUDE Hooker Electrochemical Co., Niagara Falls, N. Y .
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NVESTIGATIOKS on a laboratory scale of vapor phase reactions such as hydrogenation and chlorination, as well as many other reactions, require a controlled flow of liquid in the range of 0.1 t o 10 ml. per minute over lengthy periods of time. A complicating factor in maintaining a steady flow may be changes in back pressure due, for instance, t o changes in a catalyst bed. Many flow control devices have been developed, but all have limitations or defects. The device described here has proved particularly useful and adaptable. Plunger and gear pumps have a large field of utility, but they are particularly susceptible to damage by corrosion, and are consequently undependable for slow rates. Valves on plunger pumps frequently fail t o work satisfactorily if the pump is unduly slowed down. They sometimes become a source of metallic contamination. I n some cases displacement of the reactant b y a nonmiscible inert liquid solves the corrosion problem of the pump but in many cases is not practical. A plunger pump is apt t o deliver in pulsations, but generally can be adjusted for dropwise delivery. Control through a glass stopcock can be improved by filing tapered notches in the face of the plug and also b y extending the handle. But even this method is unsatisfactory for accurate control, owing t o the difficulty of resetting t o give a n identical flow and because a n almost infinitesimal solid speck or maldistribution of the lubricant may completely throw off the calibration. The use of capillary tubes under a controlled head is fairly satisfactory. The tube should be in a thermostatic bath such as an ice bath, as even slight changes in viscosity cause a dominating change in flow. The flow is very subject t o variation b y any trace of solid matter. Some adjustment is attainable by control of pressure, but t o change the flow substantially the tube needs t o be changed, which is a time-consuming and bothersome operation. Preparation of interchangeable tubes giving different desired flows is also not easy.
Devices based on displacement by a controlled flow of gas are subject t o changes in pressure and temperature. Displacement by a plunger device through a packing gland by means of a screw and gears is very satisfactory for some purposes, but cumbersome, and may lead t o metallic contamination. The device described is really a modification of that of Michaeli (1). T h e aim in making the modifications was t o simplify construction and add flexibility. T h e greater range of flows and accuracy in calibration was made possible by lengthening the controlling element, which is located in a more convenient position than in his apparatus. A thermostatic bath was added t o permit day and night operation under conditions of varying atmospheric temperature. An incidental advantage was observed in having an upward flow through the controlling element, in t h a t minute amounts of solid would settle before reaching it. I n Figure 1, A is a graduated t a p funnel. The glass rod or sealed glass tube or wire, E, makes a plunger which fits into D and can be slid u p and down in it so as t o permit exactly the required flow through t h e annulus Letween them. At F a combination of rubber tubes is used t o make a tight seal on both D and E but permit E t o be moved. Cylinder B standing in an overflow cu forms a thermostatic bath, holding ice water, for example. is a capillary tube about 1-mm. bore, forming a pressure equalizer and holding head h constant irrmpective of minor changes in back pressure and liquid level. If a tube of lar er diameter is used, an oscillating flow develops under some confitions. The flow can be observed and drops counted in the adapter! I , which feeds into the reactor. Usually the liquid has t o vaponze on entering the reactor and condensate is apt to reflux in this adapter. Some gas must be fed through H t o prevent this. Usually the gas is a reactant; if there is no aseous reactant a few bubbles a minute of nitrogen can be fed in. &are must be taken that no air is trapped in the tubing under E and that stopcock J is open when this plunger is manipulated, thus avoiding sucking in gas. An almost infinite ran e of flow rates is attainable by selecting suitable sizes for D a n f E . T h e effective head, h, is suitabIy about 3 inches. If it is much
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