Rust-Prevention Issues with THI®F-502 PAG Quenching Fluid

(1) Solution for Rusting of Fixtures or the THI®F-502 Quench Tank

Solution:

THI®F-502 PAG quenching fluid contains rust-inhibiting additives, and its pH is above 8.5 when leaving the factory. If long-term maintenance is neglected, the quenching solution may become neutral or acidic. You can use pH test paper to check the pH value. Rusting of fixtures or the tank is often caused by the solution entering an acidic condition. If testing shows an acidic pH, the following corrective actions can be taken:

a. Add an appropriate amount of sodium hydroxide flakes (NaOH) to adjust the solution back to an alkaline environment. A pH range of 8.5–9 provides optimal rust-prevention. The specific method of addition is provided on the next page.

b. Add THIS-629, a dedicated biocide for quenching fluids. After sterilization, the pH will increase.

(2) Solution for Rusting of Workpieces After Quenching

Solution:

If the quenching fluid concentration falls below 5%, and the workpieces have high rust-prevention requirements (e.g., in the hardware tool industry), rust inhibitors may be required.

① Test whether the quenching solution has become acidic, and correct it using the methods described below.

② Adjust the formulation of the PAG quenching solution based on rust-prevention requirements by adding Rust inhibitors to improve performance.

③ If the requirement still cannot be met, apply a separate rust-prevention process after quenching, such as using rust-preventive agents or rust-preventive oil.

④ Instructions for using pH test paper:

pH test paper (range 5.5–9) is available at chemical reagent stores for 3–5 RMB per strip. Once the pH drops below 9—especially when becoming neutral or acidic—rusting becomes more likely. Industrial sodium hydroxide flakes (NaOH, ≥99% purity) can be added to the quenching solution for correction. NaOH is sold in chemical stores in 25 kg bags at about 3–4 RMB/kg.

⑤ If laboratory conditions are available, take 500 g of working quenching solution in a beaker and measure its pH. If it is below 9, add a small amount of NaOH flakes into the beaker, stir, and measure again. Repeat until the pH reaches 9. Divide the amount of NaOH added by 500 g to determine the required percentage, then calculate the amount needed for the entire quench tank. Dilute the NaOH slightly with water before adding it to the tank. After thorough mixing, measure the pH again.

⑥ If laboratory conditions are not available, the NaOH flakes may be diluted with water and added directly using a gradual, incremental approach. Do not add the powder directly. Do not overdose—pH should be adjusted only to 9. Excessively high pH increases cooling speed and may affect performance.

⑦ Once the pH is adjusted to 9, the solution will no longer be prone to causing rust under normal conditions.

Tags:Rust Prevention, PAG Quenchant, Corrosion Issues, Quenchant Concentration, Anti-Rust Protection, THI F-502, Honorhonest, Heat Treatment Corrosion