Medium Temperature Phosphating Technology in Plastic Processing of Steel Pipes

Literature Overview

The paper by Chu Xu, Peng Yang, He Yuan, Zhou Xianmin, Wu Fusheng, and Zhou Yu from Shenyang Paka Seimitsu Co., Ltd., published in Materials Protection (Vol. 54, Issue 2, 2021, pp. 104–108), presents a systematic investigation into the application of medium-temperature phosphating for steel pipes undergoing plastic deformation processes. The authors employed SEM, XRD, salt-spray/humidity testing, copper sulfate drip test, and strip-off method to evaluate the corrosion resistance of phosphated steel pipes and to determine the optimal process window. The work is particularly relevant for engineers involved in cold forming, bending, and roll-forming operations where surface protection prior to forming is critical.

Core Technical Content and Process Parameters

The study identified the following optimal phosphating parameters:

Under these conditions, the resulting phosphating film exhibited a coating weight of up to 7.42 g/m² and an average film thickness of 3.0 μm. The primary crystalline phase was identified as Zn₂Fe(PO₄)₂·4H₂O, consistent with the zinc-iron phosphate system expected at medium temperatures. The copper sulfate drip test demonstrated corrosion resistance exceeding 300 s, a significant improvement over unphosphated or low-temperature phosphated counterparts.

Technical Analysis and Engineering Insights

Phosphating Chemistry and Film Formation

Medium-temperature phosphating, typically conducted in the 60–80 °C range, occupies a strategic middle ground between low-temperature phosphating (20–50 °C) and high-temperature phosphating (80–100 °C). At 70 °C, the chemical reaction kinetics are sufficiently rapid to form a dense, well-crystallized coating within a short immersion time of 10 minutes. The total acidity and free acidity ratio is critical; a free acidity of 4.2–5.4 mg/L within a total acidity of 30–40 mg/L ensures that the solution maintains adequate activity for nucleation while avoiding excessive dissolution of the steel substrate. The accelerator, typically a nitrate or nitrite salt, reduces the induction period of film formation and promotes uniform nucleation across the pipe surface.

Film Morphology and Corrosion Performance

The SEM and XRD results confirm that the optimal process produces a film with fine, uniformly distributed crystals. This morphology is essential for two reasons: first, it provides a continuous barrier that inhibits the direct contact between the steel substrate and corrosive media; second, it offers a favorable surface texture for subsequent paint adhesion, which is a common downstream process in steel pipe manufacturing. The copper sulfate drip test result of over 300 s indicates that the phosphating film effectively delays the onset of localized pitting, which is particularly important for the internal surfaces of pipes that may be exposed to moisture during storage and transportation.

Implications for Plastic Processing

The primary engineering motivation for phosphating prior to plastic deformation is to reduce friction and wear during forming operations. The phosphating film acts as a solid lubricant during cold bending, roll forming, and stamping. However, the film must be sufficiently robust to survive the strain imposed during forming without cracking or flaking. The 3.0 μm thickness achieved under optimal conditions represents a balance: thick enough to provide lubrication and corrosion protection, yet thin enough to avoid excessive brittleness that could lead to film fracture under high strain.

Common Defects and Countermeasures

Study Insights and Engineering Recommendations

This work reinforces the importance of process parameter optimization in surface treatment for steel pipes destined for plastic forming applications. The study's methodology—combining electrochemical testing, morphological analysis, and practical corrosion testing—provides a robust framework that can be adapted for other surface treatment processes such as chromate conversion coatings or organic coatings. From a quality control perspective, I recommend that engineers implement in-line monitoring of total acidity, free acidity, and accelerator concentration using portable pH meters and titration kits, as deviations outside the specified ranges will directly impact film quality and downstream forming performance. The 70 °C operating temperature also warrants consideration of energy consumption and safety protocols, particularly in continuous production lines where thermal management of the phosphating bath is essential for consistent quality. Overall, this study provides actionable process guidance that can be directly integrated into production specifications for steel pipe manufacturers seeking to improve both formability and corrosion resistance.