Estimating the Corrosion Rate of Mild Steel in Sulfuric Acid by a Hydrogen Evolution Method .
A. R. Basman. G. B. Chorlev.. and S. B. Adeloiu Electrochemical and Corrosion Technology Research Group, Department of Chemistry University of Western Sydney, Nepean, PO Box 10, Kingswood, NSW 2747 Without exaggeration the progress ofmodem civilization relics hcavily un the use of mctald. Unfortunately, thcsc substances can c o r n ~ d rin vuriou.i envinmments duc u, chemical or electrochemical reactions. The corrosion of metal has various undesir:ible consequences: irretricvable loss of the metal, damage of complex and expensive equipment, deterioration of'product quality, environmental pollution, and others. Evidently, the understanding of corrosion processes is important, particularly in improving the standard of corrosion orevention. A eood examole of the ~ r o b l e mis the corrosion of steel in aqueous acid media.
-
The Corrosion of Iron-Based Alloys The corrosion of iron-based allovs in acid solutions is important in industrial processes. 1t"usually causes the depolarization of hvdroeen. The two main Drocesses res~onsible for this reaction are: 'the oxidation of iron Fe + Fe"
+2 e
(1)
the reduction of hydrogen ions 2Ht + 2C + H2(g)T
(2)
These two half-reactions indicate that the electrons released in the oxidation process are assimilated by the hydrogen ions in the solution, resulting in the evolution of hydrogen. Summing these two half reactions gives Fe + 2H' + Fez++ H2(g) f (3)
mild steel coupon with a hole for suspending on a glass hook analytical balance reading to 0.1 mg thermometer barometer stopwatch Procedure The apparatus for measuring the amount of hydrogen evolved is illustrated in the figure. The Buchner flask is filled with 5M HzS04(about 300 to 350 mL) to a marked level so that the steelcoupon is fully immersed. Before be~ n n i n gthe , student must determine two things: the initial ;olume(~,) of air between the mark and the Gvel of water in the manometer; and the cross-sectional area of the glass tube in the manometer. Abrade the sample by wet-polishing,' rinsing with water, and drying with clean tissue paper. Then weigh. Open the tap, and suspend the sample . by . the glass hook. Insert the sample into t h e solution, and press the adaptor. Shut the tap, and fur the spring to prevent leaking- of gaseous hydrogen. Start the ~ & ~ w a t c h : Measure the atmospheric pressure and temperature. Take a reading of A1 after about 15 to 20 min. Then immediately remove the sample from the flask, and rinse well. Remove the corrosion product on -the surface of the steel with a toothbrush. Rinse the sample again, and then dry and weigh it. Calculate the weight loss. The corrosion product removed is mainly iron(I1) sulfate, which is the secondary product of the reaction. I n spite of its high solubility in the solution, the sulfate can partly
If the first half-reaction (eq 1)proceeds a t a higher rate than the second one (eq 2), the corrosion potential (E,,,) will move to more negative values. If the second half-reaction is faster, the E,., will move to more positive values. However, when conditions are such that both half-reactions oroceed a t the same rate. the E.- . will remain constant. This occurs for a system consistingof an active stwl and sulfuric acid. In this case. the number of' moles of iron(II1 ions formed will equal the number of moles of hydrogen molecules evolved. Such a system thus provides an ideal basis for estimating the corrosion rate of steel by measuring t h e amount of hydrogen evolved i n acidic media. Experimental Materials manometer U-tube (diameter: about 6 rnm: height: about lm) filled with colored water ' Hucnrwr flask about 500 m l I wth ground glass neck cunt \crew thread ad~ptor uith glass hook affixed' to the ~ n ternal side of the cone .tap
plastic tubes for connections sealant2to make the connections gas tight 'Araldite epoxy resin (5 min) is recommended 'window and glass silicone can be used.
258
Journal of Chemical Education
Apparatus for measuring the amount of evolved hydrogen. 1 - Buchner flask ; 2 - water manometer; 3 - tap; 4 - sample of interest; 5 glass hook; 6 - adaptor; 7 -spring.
Comparison of the Calculated and Measured Corrosion Weight Losses and Corrosion Rates for Three Steels Steel
Calculated Weight Loss (grams) Fe
Mild steel 5403 Cr13 5430 Cr17
Cr
0.0874
-
0.036 0.0150
0.0036 0.0019
Total
Measured Weight LOSS (grams)
Time of Corrosion Test (min)
Specimen Surfacf$Area
0.0881 0.0395 0.0168
16 17 20
1.596~10" 1.090 x 10" 1.098~10"
0.0874 0.0402 0.0169
precipitate on the surface of the sample due to oversaturation of solution near the surface area. This oversaturation is caused by retardation of product removal from the surface into the bulk of the solution.
Treatment of Results The gas occupying space between the test solution and the water in the manometer is a mixture of ambient air and the hydrogen that evolved during the corrosion process. Acwrding to Dalton's Law, the pressure of the gas is the sum of partial pressures of air (Pair)and hydrogen (Phyd).
where Pamis the atmospheric pressure; and Sgh is the hydrostatic pressure of water in the manometer (h = 2AZ). Calculations Usmg the isothermle gas law, calculate the pamal pressure of ~ iPa,,, r from the fulluwinl: exprewon:
where V, is the initial volume of air; andA is the cross-sectional area of the glass tube in the manometer. (Both values are provided ta students.) Calmlate Phydfiwm eq 4 . Using the gas law, calculate the number of moles of hydrogen evolved (19)from the following expression:
where 6 is the number of moles of hydrogen evolved; R is the gas constant: and T is the absolute temperature. Calculate the we~ghtloss afisumlng that the m~mhrrof moles ufwnired Iron and the numher of molesofevolved hydmgen are the same: Am = M6
(7)
where M is the atomic weight of iron (in grams). Stainless Steel I n addition to mild steel, samples of stainless steels (e.g., SS403 with 13% Cr and SS430 with 17% Cr) can be used. The experimental procedure is the same a s that for mild steel. The method of weight loss calculation3 is slightly different because a corresponding part of hydrogen is evolved
Corrosion Rate Corrosion Rate Based on C.W. Based on MW (g m-'day-') (g .m-'day-')
.
(m )
4.916~lo3 4.954 lo3 3.073 x lo3 3 . 0 2 0 ~lo3 1.105~10~ 1.102~10~
due to the oxidation of chromium through the following reaction. Cr + cr3++ 3, (8) I n this case, the total weight loss must be calculated by the following equation: Am =
1.:
)
MiPiKi
(9)
where n is the number of alloying elements in the steel; Mi is the atomic weight of element i; Pi is the fraction of element i in the steel; K j is the number of moles of element i equivalent to 1 mol of molecular hydrogen; and 6 is the number of moles of evolved hydrogen. For chromium stainless steel n = 2. Far 55403 the Pi of chromium is 0.13, and the Piof iron is 0.87. F o r iron K = 1, and for chromium K = 2\3. Calculations Compare the calculated weight loss with the measured one. Determine the corrosion rate in units of g . m-' dapl on the basis of the calculated and measured weight losses.
Discussion This experiment was designed to demonstrate the connection between the anodic reaction (oxidation of a metal) and the cathodic reaction (reduction of hydrogen ions), thus enhancinz .. the students' understanding.. of corrosion a s an electrochemical procesn. Also, treatment of the test results involves using - gas - laws. which students often find difficult to apply. Unlike the apparatus used in most studies of electrochemical coms'ion, the apparatus is very simple to useand docs not include complicated electronic equipment. The experiment is suitablefor university students a s well a s for high school students. Typical experimental results are given in the table, which shows that this experiment can be used to demonstrate the beneficial effect of alloying steels with chromium. Furthermore, it is possible to adopt this experimental approach to study how various other factors affect the corrosion rate of steel. For example, new studies can be done by varying the solution corrosivity, using different pH's, or by varying the concentration of some inhibitor, such as hydrogen peroxide. General References
Tne calculat on must be carr ea out assum ng no seledlve leachng, whlch 1s the case tor tne a d ve sta nless steel in sult~r c acld.
Volume 70
Number 3 March 1993
259
