Introduction to Corrosion Coupon Tests in the Petrochemical Industry

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The corrosion test (patch) method is widely used as an important means to detect and monitor the corrosion of specific materials or media. The accuracy of the data obtained through this method has a direct impact on the results of related research as well as on the effectiveness of corrosion prevention measures, such as those related to corrosion inhibition studies and evaluations. As an important part of this method, the quality of the corrosion test specimens has a significant impact on the data. Corrosion test coupons are hung inside the testing container or on heat exchangers or test pipes for on-site testing, and serve as standard metal coupons for measuring corrosion. Used to detect or evaluate the corrosion condition of the cooling water system. Generally, the pre-treated and weighed metal specimens are placed in the testing system for a period of time (such as 30 to 90 days); alternatively, the same set of specimens can be taken out at different times, such as after 30 days, 60 days, 90 days, etc. During long-term observation, 12 or 24 test pieces are placed each time; 1 or 2 pieces are removed monthly, cleaned and weighed to determine the degree of metal corrosion loss (in mils per year, where 1 mpy = 0.0254 mm/year). The type of corrosion as well as the depth of pitting are also determined in order to assess the corrosion rate, which is then plotted as a curve showing corrosion rate over time. 　 The monitoring period for corrosion test pieces is generally 30 to 90 days; it is also possible to retrieve the test pieces from the same group at different times, such as after 30 days, 60 days, 90 days, etc. During long-term observation, 12 or 24 test pieces are placed each time, and 1 or 2 pieces are taken out monthly to measure the corrosion rate respectively. Subsequently, the corrosion rate versus time curve is plotted. Processing execution standard: HG/T 3523-2008 – Standards for chemical treatment of cooling water; Technical specifications for corrosion test pieces. Application areas: Suitable for water quality management and research on water treatment agents, as well as for corrosion research and monitoring in other systems. Common materials: A3 carbon steel test pieces, 20# carbon steel test pieces, stainless steel test pieces, brass test pieces, copper test pieces, aluminum test pieces, cast iron test pieces, etc. Common specifications: Type i 50×25×2mm, Type ii 72.4×11.5×2mm ； Type III standard corrosion test coupons, dimensions: 40×13×2 mm. Specifications for corrosion testing coupons: Type I, dimensions 50×25×2 mm. Methods of storage and use: 1. Wash the standard corrosion test coupons with a degreasing agent in distilled water, then rinse them with distilled water for 15 seconds ； 2. Clean the standard corrosion test coupons (corrosion measurement specimens) with a degreasing agent in chemically pure anhydrous ethanol (50 ml per 10 coupons) twice ； 3. Place the standard corrosion test coupons on dry filter paper and dry them with cold air ； 4. The standard corrosion test coupons (for measuring corrosion) should be wrapped in filter paper and placed in a desiccator; after 24 hours, they are weighed for use. 5. Place the originally packaged test samples in a location free from corrosive gases, and store them in a moisture-proof environment ； 6. The opened but unused standard corrosion test coupons (corrosion testing strips) should still be placed in rust-proof paper and kept in a desiccator. 7. The installation location of the hanging fixture can be determined based on the testing objectives. Areas with the most severe corrosion and representative dissolved oxygen levels should be given priority. 8. Attention should be paid to the hanging direction of the test pieces in the system, so that corrosion products do not fall on adjacent test pieces. 9. The hanging position of the test pieces should be relatively fixed, ensuring that the test pieces in the same area are under identical environmental conditions. 10. During installation, it is not allowed to touch the mounted test pieces with hands or any other dirty substances. Treatment of new test strips (standard test strips in sealed packaging): 1. Immerse the new test strips in anhydrous ethanol and wipe them twice with absorbent cotton ； 2. Then transfer it to clean anhydrous ethanol and soak it for a while ； 3. Place on clean filter paper and dry with cold air ； 4. Wrap it in filter paper, place it in a desiccator, and weigh it after 24 hours for use (weigh to 0.1g precision). Treatment for used hanging slices: 1. Carefully remove the used hanging slices, observe and record their surface condition before proceeding with further treatment ； 2. For those cases where corrosion deposition is not obvious, wipe it with a drawing eraser to reveal the natural color of the metal. Then immerse it in anhydrous ethanol and scrub it twice with absorbent cotton; after that, soak it in clean anhydrous ethanol for a while. Take it out, place it on clean filter paper, dry it with cold air, wrap it in filter paper, and store it in a desiccator. Weigh it after 24 hours (to an accuracy of 0.1 g) to determine the weight loss, and calculate the corrosion rate ； 3. For test pieces with a large amount of corrosive deposits, the corrosion products should first be removed using chemical cleaning methods. The preparation and usage instructions for common chemical cleaners are shown in the table below:

| Test piece material | Cleaning agent components | Cleaning time | Temperature | Instructions |
|---------------------|--------------------------|---------------|-------------|-------------|
| Carbon steel | 10% hydrochloric acid + 0.5% hexamethylenetetramine (or 7019) | <20 minutes | Room temperature | Wash while gently scrubbing; results are poor at temperatures below 15°C |
| Copper and copper alloys | 15–20% hydrochloric acid or 5%–10% sulfuric acid | 3–5 minutes | Room temperature | Wash while gently scrubbing |
| Aluminum and aluminum alloys | 8% phosphoric acid + 2% chromic anhydride (CrO3) or 70% nitric acid | 6–10 minutes | 80°C for 1 minute | Then at room temperature | An oxide film remains on the surface |
| Stainless steel | 10% nitric acid or 15% ammonium citrate | 10–20 minutes | 10–20 minutes | At room temperature or 70°C |

After chemical cleaning, the test pieces should be immediately immersed in a 5N sodium hydroxide solution for passivation for a short period. After that, they should be placed in clean anhydrous ethanol, dried with filter paper, and then rubbed with an eraser to reveal the metal’s natural color. The subsequent treatment steps are the same as those described in point 2 above. 　Measurement of corrosion rate: The corrosion rate of the metal is calculated using the total surface area of the test piece, the density of the metal, the duration of the test, and the weight loss of the test piece, according to the following two formulas:
Corrosion rate = 87.6△W/(Sρt) mm/year
Corrosion rate = 3449 △W/(Sρt) mpy
Where △W represents the weight loss of the test piece, in mg ； S — Total surface area of the test piece, cm2 ； 　　ρ —— density of the metal, g/cm. (for example: carbon steel 7.85, stainless steel 7.92, brass 8.50, aluminum 2.70) ； 　　t — test time, h.