Common troubleshooting methods for steel phosphating
During the steel phosphating process, various faults often occur due to improper process parameter control, changes in phosphating solution composition, and incomplete pretreatment, affecting the quality and performance of the phosphating film. Common faults include coarse phosphating film crystals, loose film layers, partial absence of film, film smearing, and rust. These faults not only reduce the corrosion resistance and coating adhesion of the phosphating film, but may also cause problems in subsequent processes. Therefore, timely diagnosis of the cause of the fault and implementation of effective troubleshooting measures are crucial.
Coarse crystallization of the phosphate coating is a common problem, manifesting as a rough surface, distinct crystal particles, and, in severe cases, a powdery substance. The main causes of this problem include excessive phosphate solution concentration, high free acidity, excessive treatment temperature, or prolonged treatment time. Excessive phosphate solution concentration can cause excessive phosphate crystal growth, resulting in coarse crystals; excessive free acidity can exacerbate dissolution of the steel surface, causing uncontrolled crystal growth; and excessive temperatures or prolonged treatment times can lead to excessive crystal growth, resulting in a rough coating. Troubleshooting methods include adjusting the phosphate solution concentration, reducing the free acidity (by adding zinc oxide or zinc hydroxide), appropriately lowering the treatment temperature or shortening the treatment time, and, if necessary, replacing part of the solution to reduce impurities.
A loose film is another common problem, manifesting as a loose phosphate film that easily falls off. The powder can be easily wiped off with a finger. Causes of a loose film include low free acidity, high total acidity, low temperature, insufficient treatment time, or excessive impurities in the phosphate solution (such as excessive Fe²⁺ concentration). Low free acidity slows the phosphating reaction and results in incomplete film formation; high total acidity leads to rapid film growth and a loose structure; low temperature or insufficient treatment time results in incomplete phosphating; and high Fe²⁺ concentration interferes with the proper formation of the phosphate film, resulting in a loose film. Troubleshooting can be achieved by appropriately increasing the free acidity (by adding phosphoric acid), reducing the total acidity (by adding water for dilution), increasing the treatment temperature, or extending the treatment time. For high Fe²⁺ concentrations, an oxidant (such as sodium nitrite) can be added to oxidize the Fe²⁺ to Fe³⁺, which then precipitates and removes it.
Localized absence of film or discontinuous film is also a common fault, manifesting as the absence of phosphate film on some areas of the workpiece surface, or a very thin film, exposing the base metal. The main reasons for this fault are incomplete pretreatment, oil, rust, or scale on the workpiece surface that has not been cleaned ; clogging of the phosphating solution nozzle, resulting in partial contact of the workpiece with the phosphating solution; excessive smoothness of the workpiece surface, which is not conducive to the adhesion of the phosphating film; insufficient concentration of accelerator in the phosphating solution, and too slow reaction speed. The solution to eliminate this problem is to strengthen pretreatment to ensure that the workpiece surface is free of oil and rust; check the nozzles of the phosphating tank, clean any clogged nozzles, and ensure uniform spraying of the phosphating solution; for workpieces with smooth surfaces, the pickling time can be appropriately increased to improve the surface roughness; and adding accelerators (such as sodium nitrite or sodium chlorate) to speed up the phosphating reaction.
Film blooming manifests as uneven color on the surface of the phosphate film, with spots or streaks of varying depths, affecting the appearance quality. Causes of this failure include uneven phosphating solution concentration, excessive fluctuations in the processing temperature, residual pickling agents or degreasing agents on the workpiece surface, and excessive impurities in the phosphating solution. Uneven phosphating solution concentration can lead to different local reaction rates, resulting in blooming; excessive temperature fluctuations can affect the uniformity of crystal growth; residual agents can interfere with the phosphating reaction, leading to local film abnormalities; and excessive impurities can affect the normal formation of the phosphate film. Troubleshooting methods include increasing the stirring of the phosphating solution to ensure uniform concentration; stabilizing the processing temperature to avoid large fluctuations; increasing the number of water washes to thoroughly remove residual agents on the workpiece surface; regularly cleaning the sediment and impurities in the phosphating solution, and replacing part of the phosphating solution when necessary.
Rust recurrence is a common problem caused by improper post-phosphating treatment, manifesting as rust on the phosphate film surface during drying or storage. Main causes of rust recurrence include incomplete post-phosphating rinsing, which allows residual phosphating solution to corrode the workpiece after drying; insufficient or inadequate post-treatment passivation; incomplete drying, which allows residual moisture on the workpiece surface; and storage in a humid environment with high humidity. To address this, increase the number of washes to ensure no residual phosphating solution remains on the workpiece surface; strengthen the passivation treatment to ensure the integrity of the passivation film; increase the drying temperature or extend the drying time to ensure a dry workpiece surface; and store the phosphated workpiece in a dry, well-ventilated environment, avoiding moisture.
The causes of failures during the steel phosphating process are complex and diverse. Accurately identifying the cause and implementing targeted troubleshooting measures is crucial, based on actual production practices. This involves testing phosphating solution parameters, observing fault symptoms, and analyzing the process. Furthermore, strengthening routine maintenance of the phosphating solution and monitoring process parameters can effectively prevent failures and ensure the stability of the phosphating film. With the continuous advancement of phosphating technology, the application of automated detection and control systems will further improve the efficiency of fault diagnosis and troubleshooting, providing strong support for the stable production of steel phosphating processes.
