Solve adhesion problems on high carbon steel

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Q. We handle a wide variety of part types, and lately we have had grip issues when plating our high carbon and heat treated fasteners. What can we do to alleviate this problem?

A. Adhesion issues can be common on high carbon and heat treated steel, and one of the main areas to focus on in addressing this issue is the acid pickling step of the process. High carbon steel can generally be classified as an iron-carbon alloy, ranging from 0.55 to 1.40 percent carbon. High carbon steel is harder and stronger than ordinary mild steel, and it is often heat treated to improve properties such as impact resistance, hardness, ductility and various others, depending on the type. heat treatment. The resulting composition of the substrate has a direct bearing on the performance of acid pickling.

The goal of acid pickling is to remove scale and metal oxides that typically form on the metal surface during the heat treatment process. The oxide layer is “etched” by the acidic pickling solution, producing a mixture of metal salt and water. However, a problem arises if the entire oxide layer is removed, as then the base metal would start to dissolve, producing a metal salt which releases hydrogen into the acid stripper. This is where the real challenge lies in plating high carbon and heat treated fasteners. Often higher concentrations of hydrochloric acid or alternatives such as ammonium bifluoride are required to properly remove metal oxide scale or carbon. If the oxide layer is completely removed and the base steel is attacked, the dissolution of the steel substrate creates a carbon-rich surface. The grip issues you mention are due to the inability to initiate plating on this carbon rich surface. In addition, the release of hydrogen can also lead to hydrogen embrittlement issues, a major concern in the fastener industry.

The unique composition and requirements of high carbon and heat treated fasteners present an extremely difficult scenario in finding the perfect balance between over-stripping and under-stripping. The most common solution for this problem is the implementation of an acid pickling inhibitor. Acid stripping inhibitors work by creating a protective film through adsorption methods, which protects the steel substrate but allows the oxide layer to dissolve steadily. The photo on this page shows, from left to right, a type 10.9 bolt as is, the same bolt after pickling without inhibitor and finally the bolt after pickling with an acid inhibitor. The bolt as is gives an accurate representation of a heat treated fastener and the amount of heat treated scale present. The inhibitor-free stripped bolt demonstrates the effect of a typical pretreatment cycle: it is extremely black and carbonaceous due to the resulting carbon-rich surface. Finally, the bolt with the acid inhibitor is a perfect bronze gray because the metal oxide layer has been dissolved but the substrate has not been affected. This led to effective bolt plating.

Acid pickling inhibitors offer a myriad of benefits in many different plating applications. Specifically, for high carbon and heat treated steel, they are designed to inhibit attack on the steel substrate but allow the rate at which they remove scale or other compounds to remain the same. This allows the substrate to remain relatively unaffected by the acid pickling while retaining the necessary cleaning functions of the pickling process. In addition, the life of acid pickling is generally extended, as less metal is dissolved in the pickling solution, which reduces the entrainment of any metal contamination. Acid stripping inhibitors also often contain other types of surfactants that help create a foam blanket designed to reduce acid fogging in the workplace.

So, as we have pointed out here, adhesion problems with high carbon and heat treated fasteners are often due to parts coming out of the pickling process with a rich carbon surface due to the attack of base steel. Paying special attention to the acid pickling step of the process and adding an acid pickling inhibitor can help alleviate this problem. Recent developments in acid stripping inhibitors have also reduced the excessive film formation or difficult to remove substances that were prevalent in older generation inhibitors causing staining and blistering problems. It should be noted, however, that not all acid inhibitors are created equal and may not work the same on the same substrate. It is therefore important to rely on a trusted chemical supplier to answer specific applications or questions.

Matt Schario is Director of Technical Services at Columbia Chemical. Visit columbiachemical.com.

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