Key Considerations When Using a Room-Temperature Phosphating Solution
Understanding the Classification and Techniques of Room-Temperature Phosphating Solutions
Phosphating processes are typically categorized based on the composition of the solution, with iron and zinc-based systems being the most common. However, when it comes to room-temperature phosphating solutions, this classification can sometimes lead to confusion. For example, the same solution may be used for both steel and zinc substrates. The key factor in room-temperature phosphating lies in the selection and development of oxidizing agents, which significantly affect the performance of the process. Common types of room-temperature phosphating solutions include nitrite-based (containing nitrates, chlorates, or nitro compounds) and molybdate-based systems. Molybdate solutions are widely used for metal surface treatment due to their effectiveness and stability.
Controlling Film Color
The color of the phosphating film is influenced by factors such as the type of metal ions present, the kind of oxidizing agent used, film thickness, and crystallization state. Some believe that adjusting the oxidizing agent can control the color, while others argue that different metal ions result in varying colors. However, these theories don’t always align with practical results. In real-world applications, even within the same system, differences in time or conditions can lead to color variations. Achieving the desired color without compromising corrosion resistance remains a challenge that requires careful optimization.
Temperature Control in Phosphating
Room-temperature phosphating is typically carried out between 15°C and 35°C. If the temperature exceeds 35°C, heating is usually required, while below 15°C, the dissolution rate of metals slows down, leading to longer processing times. Adjustments to the oxidant concentration may be necessary to maintain efficiency. Since room-temperature phosphating is a film-forming process, it is highly sensitive to ambient temperature, requiring strict control of process conditions. This makes it more suitable for small batch production. Additionally, the quality of the resulting film is generally lower compared to medium-temperature phosphating methods.
Sedimentation and Slag Management
In nitrite-based phosphating systems, sedimentation often occurs during use, which can adhere to the workpiece surface, interfere with film formation, and reduce coating quality. To address this, process optimization is essential to minimize slag formation, and existing sludge should be removed and properly recycled. Various methods, including separation, active sedimentation, and mechanical removal, can be employed to effectively manage sludge and improve overall results.
Yellowing and White Powder Formation
After phosphating, some surfaces may develop a powdery layer, affecting both appearance and film integrity. This issue is often caused by low free acid levels, excessive oxidizer, or sediment buildup. By addressing these root causes through proper adjustments, the occurrence of white powder can be significantly reduced.
Phosphating Different Metal Substrates at Room Temperature
While the phosphating of zinc and steel follows similar principles, the process for aluminum differs. Adding SiF₆²⻠or BF₄⻠to a zinc phosphating solution allows for effective treatment of aluminum and its alloys, producing a film primarily composed of Zn₃(PO₄)₂·4H₂O. Fluoride compounds like NaF or NaSiF₆ are commonly added to room-temperature solutions. However, Al³⺠acts as a negative catalyst, and when its concentration exceeds 0.5 g/L, it can cause fading, unevenness, or complete failure in film formation.
Summary
Recent advancements in multi-metal phosphating have gained significant attention, especially with increased fluoride content. Solutions containing ammonium hydrogen phosphate, ammonium fluoride, molybdenum or tungsten salts, zinc complexing agents, NaBF₄, sodium nitrobenzenesulfonate, and zinc nitrate can successfully treat aluminum, zinc, and steel. With the growing diversity of automotive materials—such as steel, galvanized steel, and aluminum—developing a room-temperature phosphating solution capable of handling multiple substrates simultaneously has become a key research direction.
[Source: Hefei Xianghe Phosphate Coating Co., Ltd. http://]
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