In many stages of industrial manufacturing, reliable support from fasteners is indispensable. Every bolt and nut plays a critical role in connecting components and bearing loads. As a vital surface treatment step in fastener production, the blackening process forms a uniform and dense black oxide layer — not only enhancing appearance, but more importantly, serving as a key barrier against corrosion.
Unstable Blackening: A Common Industry Challenge
In real-world production, issues such as patchy coatings with exposed metal, uneven blackening, brittleness after minor impacts, and even rust protection failure are all too common. The blackening bath may also develop microbial growth and unpleasant odors. These problems not only compromise product quality but also lead to high maintenance costs, frequent tank cleaning, and increased environmental pressures — making them some of the most widespread pain points for fastener manufacturers during the blackening process.
Image | Common Pain Points in the Fastener Blackening Process
KERUN’s Specialized Formula: Tackling the Problem at Its Root
To address the above challenges, KERUN — a leading provider of green industrial fluid system solutions for the fastener industry — proudly recommends the PR520 Residual Heat Blackening Agent. In residual heat blackening processes, the heat retained by the workpiece after heat treatment drives moisture evaporation, which in turn triggers resin deposition from the blackening emulsion to form a coating on the surface. However, if this process is unstable — such as uneven evaporation or incomplete resin deposition — the result can be rough, uneven, or patchy coatings.
KERUN PR520 tackles these issues at the formulation level. By incorporating advanced resins and nano-silica additives, it significantly improves film uniformity and density. Its strong hydrophobic properties also enhance compatibility with subsequent rust preventive oils, extending corrosion protection duration. Field tests at customer sites have shown that, when used together with KERUN KR-AOD330Plus rust preventive oil, fasteners can be protected for up to one year in storage — delivering long-lasting anti-corrosion performance.
Moreover, PR520 generates minimal sludge during use and extends tank-cleaning intervals, truly meeting both demands of “high-quality blackening” and “long-term stability”. It brings greater ease, consistency, and reliability to the production line.
Image | SEM Comparison of Coating Uniformity (A: KERUN Product B: Other Brand)
Image | Real-World Results After Using PR520 with KR-AOD330Plus
Customer Case Review
One of the world’s leading manufacturers of hot-forged fasteners — a fastener plant in Jiangsu, China — had previously encountered issues such as poor adhesion and flaking of the black oxide layer when using a competing brand of blackening agent. These problems negatively impacted product appearance and downstream processing efficiency. Since switching to KERUN’s PR520 Residual Heat Blackening Agent, the plant has seen a significant improvement in film adhesion, uniformity, and overall stability. The flaking issue has been effectively resolved, resulting in a more reliable and consistent blackening quality across the board.
Black Coating Peeling with Previous Brand’s Blackening Agent
Uniform and Intact Black Coating After Using PR520
At KERUN, we are committed to being a leader in green industrial fluids, providing one-stop solutions for the fastener industry. Beyond blackening agents, our portfolio includes quenching oils, rust preventive oils, and more — ensuring that every bolt and nut meets the highest quality standards and supporting the stable, reliable operation of fastener production lines.
