Why Do 82B Wire Rods Undergo Pickling and Phosphating?

As we all know, 82B high-carbon wire rod is a primary raw material for producing prestressed steel strands and steel wires, widely used in various infrastructure projects. This type of steel possesses relatively high strength and hardness; following hot rolling and cooling, a dense oxide layer—composed of ferrous oxide (FeO) and ferric oxide (Fe₂O₃)—forms on the surface, accompanied by impurities such as rust spots and oil stains.

Failure to thoroughly remove these impurities directly interferes with subsequent processing steps such as wire drawing and cold heading; this not only results in finished products failing to meet quality standards but also accelerates mold wear, making the precise removal of the oxide layer a crucial stage in production.

Pickling and phosphating is a mature and mainstream surface treatment process for wire rods. By combining the steps of removing rust through pickling and forming a protective film through phosphating, it thoroughly eliminates surface defects and lays a solid foundation for subsequent processing.

Many manufacturers currently opt for physical rust removal processes to cut costs and streamline workflows. However, in actual production, methods such as wire brushing and abrasive belt grinding only remove surface rust; they fail to eliminate the dense, tightly adherent mill scale and deep-seated corrosion, potentially compromising product quality over the long term.

Main Rust Removal Methods:

82B wire rods treated via standardized pickling and phosphating feature a smooth, clean surface free of scale and mechanical damage, with uniform coloration throughout the strand; they meet general industry quality standards as well as the rigorous requirements of overseas projects, making them suitable for a wide range of market applications.

In contrast, wire rods processed via physical descaling typically exhibit a rough, matte surface and uneven coloration, often retaining residual scale and lacking overall substrate uniformity. Due to inherent process limitations, physical descaling suffers from unavoidable shortcomings regarding descaling cleanliness, lubrication stability, and product consistency, rendering it unable to replace the pickling and phosphating process.

For high-carbon, hard-grade wire rods such as 82B, compliant pickling and phosphating remains the core process for deep processing—offering the greatest stability and suitability while best aligning with market demands.

I. Advantages of Pickling and Phosphating

There are significant differences between mechanical descaling and the pickling-phosphating process, particularly regarding surface cleanliness, the formation of processing-compatible coating layers, and the mechanical stability of the finished product.

1. Thoroughly Remove Surface Impurities to Ensure a Clean Substrate.

The oxide scale on the surface of the wire rod is hard and strongly bonded to the substrate, making it difficult to remove via standard physical cleaning methods. Pickling utilizes hydrochloric or sulfuric acid at concentrations of 10%–25% to chemically dissolve oxide scale, rust, and oil, revealing the clean, natural metallic surface of the rod and achieving a rust removal rate of up to 98%. Following the reaction, multiple water rinsing stages are employed to completely eliminate residual acid and reaction by-products, preventing the carry-over of impurities into subsequent processing steps.

However, physical descaling methods can only remove surface rust from the wire rod; they fail to eliminate mill scale and deep-seated rust hidden within crevices. This leads to wire breakage during subsequent drawing processes, causes die wear, increases the scrap rate, and disrupts production. 

2. Formation of A Dense Phosphating Film to Enhance Processing Suitability.

Phosphating is a critical step following pickling; clean wire rods are immersed in a zinc or manganese phosphate solution, where a chemical reaction generates a dense phosphate film (such as Zn₂Fe(PO₄)₂· 4H₂O) measuring 10–15 micrometers in thickness, thereby providing essential performance support for subsequent processing. In contrast, mechanical descaling lacks a film-forming stage and cannot create a functional protective coating.

Key Characteristics of the Phosphating Coating:

- Basic Corrosion Resistance: Isolates the metal substrate from air and moisture, providing short-term rust protection and preventing the recurrence of rust.

- Strong Adsorption: The porous structure of the coating firmly adsorbs lubricants (such as metal soaps), creating a "phosphating coating + lubricant layer" composite lubrication system suitable for cold-working processes.

- Low Coefficient of Friction: With a friction coefficient of approximately 0.05, it significantly reduces friction during drawing and cold heading, minimizes die wear, and prevents surface scratching on the product.

After pickling and phosphating, the surface of the wire rod exhibits a uniform, smooth metallic luster; the phosphate coating is chemically bonded to the substrate, forming a dense and stable structure that prevents issues such as coating agglomeration or peeling. This uniformity and adhesion are crucial for ensuring consistent performance during subsequent wire drawing. 

3. Mitigating Processing Risks and Stabilizing Mechanical Properties.

For wire rods that have undergone physical descaling, residual deep-seated scale can cause stress concentrations during drawing, leading to surface cracks or wire breakage; in contrast, a phosphate coating uniformly distributes processing stresses, thereby reducing the risk of cracking. Furthermore, while pickling removes the surface oxide layer, the phosphate coating acts as a buffer against processing impact, ensuring the stability of mechanical properties—such as tensile strength and reduction of area—following the wire drawing process.

Summary

The core value of the pickling and phosphating process lies in its ability to thoroughly remove various types of rust and impurities from the surface of wire rods while simultaneously forming a functional phosphate coating optimized for cold working. It represents the optimal process choice for balancing impurity removal, processing performance, production costs, and the requirements of large-scale manufacturing. 

As a one-stop supplier of a comprehensive range of prestressing products, we place great importance on this fundamental process; we strictly control chemical formulations and oversee every stage of production to ensure consistent quality from the very source. 

Welcome to inquiries for quotations on products such as steel strands, steel wires, and anchorages, and cordially invite you to visit our facility for an on-site inspection. We look forward to the possibility of a partnership.