Determination of the corrosion rate of metals by the weight-loss method

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Experiment 1: Determination of the corrosion rate of metals using the weight-loss method
I. Objectives and requirements of the experiment
1. Understand the principles and methods of determining the corrosion rate of metals via the weight-loss method； 2. Determination of the corrosion rate of carbon steel in dilute sulfuric acid by the weight-loss method ； 3. Understand the effect of corrosion inhibitors on the metal corrosion rate, and determine their inhibition efficiency using the weight loss method. II. Experimental Principles and Methods: When metal undergoes uniform corrosion, there are generally two ways to express its corrosion rate. One method is to use the weight of metal lost (or gained) per unit area per unit of time, with the commonly used unit being g/m2•h ； Another method is to express it as the depth of metal corrosion per unit time, with mm/a being the commonly used unit. Currently, there are many methods for determining the corrosion rate of metals, such as gravimetric method, volumetric method, polarization curve method, linear polarization method (i.e., polarization resistance method), etc. The gravimetric method is one of the more classical methods; it is applicable to both laboratory and field tests. It is one of the most reliable methods for determining the corrosion rate of metals, and serves as a basis for other methods used to measure metal corrosion rates. The gravimetric method determines the corrosion rate of metals based on the change in the weight of metal specimens before and after corrosion. The gravimetric method is divided into two types: loss-on-weight method and gain-on-weight method. The loss of weight method is commonly used when the corrosion products on the metal surface can be easily removed without damaging the metal itself as a result of that removal ； The weight gain method is used when the corrosion products are firmly attached to the surface of the specimen. Metal is shaped into specimens of specific forms and sizes, which are placed in corrosive environments such as the atmosphere, seawater, soil, or various experimental media. After a certain period of time, these specimens are removed and their weight and dimensions are measured; by doing so, the corrosion rate can be calculated. For the weight-loss method, the corrosion rate of metal can be calculated using the following formula: (1-1) In this formula: v – the corrosion rate of the metal, g/m²·h ； W0 —— weight of the specimen before corrosion, g ； W1—Weight of the specimen after corrosion and removal of corrosion products, in g ； S —— Surface area of the test specimen exposed to the corrosive environment, in m2 ； t is the corrosion time of the specimen, in hours. For the weight-gain method, that is, when all of the corrosion products on the metal surface remain attached to it, or when they can all be collected after detaching, the corrosion rate can be calculated using the following formula: (1-2) In this equation: v+ —— Corrosion rate of the metal, g/m²·h ； W — Weight of the specimen after corrosion, including corrosion products, in g ； The remaining symbols are the same as in equation (1-1). For metals with the same density, their corrosion resistance can be compared using the above method. However, for genera with different densities, although the weight per unit area changes in the same way, their corrosion depths differ. At this point, it is more appropriate to express the corrosion rate of metal in terms of the corrosion depth per unit time. The conversion formula is as follows: In equations (1-3): vt — annual corrosion depth, mm/year ； r — density of the test material, g/cm3 ； v——Rate of weightless corrosion, g/m2•h. III. Experimental equipment and reagents: 2 electronic analytical balances, 1 rotating hanging slice corrosion tester, 30 rectangular carbon steel test pieces, steel grades, hammers, sandpaper, vernier calipers, anhydrous ethanol, acetone, forceps, absorbent cotton, filter paper, 10 500 mL beakers, glass rods, nylon threads, rubber, sulfuric acid (CP), thiourea (CP). IV. Experimental content and procedures The main contents of this experiment: Determining the corrosion rate and inhibition rate of carbon steel in 10% H2SO4 and 10% H2SO4 + 1 g/L thiourea using the gravimetric method. The experiments were conducted in groups of 6 people each. The experimental steps are as follows: 1. Test piece numbering ； 2. Sand the test piece with a series of sandpapers, from coarse to fine, to obtain a uniformly smooth surface ； 3. Use a vernier caliper to accurately measure the dimensions of the test piece in order to calculate its surface area (the results are listed in Table 1-1 as well) ； 4. Wash the test pieces successively with anhydrous ethanol and acetone, dry them with filter paper (the washed test pieces should be avoided direct contact with hands), and then place them in a desiccator to dry for a period of time ； 5. Weigh the dried test pieces using electronic analysis or an analytical balance, with an accuracy of 0.1 mg ； 6. Pour 150 ml of 10% H2SO4 and 150 ml of 10% H2SO4 + 1 g/L thiourea into two clean 200 ml beakers respectively ； 7. Divide the test pieces into two groups according to their numbers (three pieces in each group), and immerse them in the two beakers mentioned above respectively. The test pieces should be fully submerged, with the upper part of them about 1 cm below the liquid surface; the immersion depth of each piece should be roughly the same ； 8. Timing begins when the test piece is immersed; after 1 hour, the test is stopped, the piece is removed, washed with water, and the surface corrosion products are wiped away with an eraser. It is then cleaned with anhydrous ethanol and acetone, dried with filter paper, and placed in a desiccator to dry for a period of time ； 9. Weigh the dried test piece again using an electronic analytical or analytical balance; the accuracy should be 0.1 mg. V. Evaluation of experimental results 1. Qualitative evaluation (1) Observe the shape of the specimens after corrosion to determine whether the corrosion was uniform; also observe the color distribution of the resulting substances and whether they are firmly bonded to the metal surface ； (2) Observe whether there is any change in the color of the solution, and whether any corrosion products have precipitated. 2. Quantitative evaluation form 1-1: Data table for measuring the metal corrosion rate using the weight loss method. Date: Temperature: Experimental medium. Number, length a in cm, width b, thickness c, pore diameter f, Sm2, W0g, W1, DW, th, v–g/m2•h, I%. If it is uniform corrosion, the corrosion rate can be calculated using equation (l-1), and the annual corrosion depth can be determined using equation (l-3). The corrosion inhibition rate of thiourea under experimental conditions is calculated using the following formula: In equation (1-4): I —— Corrosion inhibition rate ； v — Corrosion rate after adding the corrosion inhibitor, g/m²•h ； v0—Corrosion rate without corrosion inhibitor, g/m²•h. VI. Experiment Report
An experiment report should include the following contents:
1. Purpose of the experiment
2. Basic principles of the experimental method
3. Analysis and discussion of the experimental results (including data and phenomena)
4. Conclusions