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Fastener Surface Treatment Process

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News

Time：

2024-10-10

Select fastener surface treatment

Many engineers prioritize selecting the appropriate type of fastener as the primary consideration in their designs, often overlooking the importance of surface treatment. When choosing the optimal surface treatment for your fasteners, it is wise to take the following factors into account:

1. Safety – Improper surface treatment can easily lead to failure of fastened joints.

2. Corrosion Protection – What is the product’s expected service life, and under what environmental conditions will it be used?

3. Preventing Damage During Operation – How do scratches and abrasions incurred during handling and installation affect surface treatment?

4. The Criticality of Connections – If a connection is loose, will the assembly fail? What are the consequences of assembly failure?

5. Functionality – Will surface treatments, due to thread or groove blockage, impede the assembly of fasteners?

6. Feasibility – Is the surface treatment readily available?

7. Cost – Can surface treatment deliver cost benefits for my assembly?

Many specialized surface coatings have been developed for specific applications, and new coatings continue to be introduced every day. This article will not attempt to cover all such coatings but will focus on the following commonly used surface treatments:

 Electro-galvanized (“commercial” galvanizing)

 Electroplated zinc-nickel

 Mechanical zinc plating

 Non-electrolytic zinc coating

 Hot-dip galvanizing

 Epoxy electrophoretic coating

Security

When selecting surface treatments for fasteners, the primary safety concern is preventing hydrogen embrittlement. Hydrogen embrittlement (HE) refers to the delayed, catastrophic failure of high-strength fasteners under tensile loading in the presence of hydrogen. Using an inappropriate surface treatment on a high‑strength fastener can lead to hydrogen embrittlement. So, what exactly constitutes the “high‑strength fastener” that is often implicated in hydrogen embrittlement?

Fasteners deemed to carry a risk of hydrogen embrittlement have a hardness exceeding 380 HV; for example, but not limited to:

 Metric property class 12.9

 Imperial‑size alloy socket‑head cap screws

 Surface-hardened, externally threaded fasteners with machined threads, such as thread-forming screws or surface-hardened flanged-head screws with knurled surfaces.

 Tapered elastic washer

 Straight-groove or rolled elastic cylindrical pins

 Retaining ring

To mitigate the risk of hydrogen embrittlement in the aforementioned fasteners, pickling and/or electroplating should be avoided whenever possible. Appropriate measures, such as immediate post‑electroplating baking, can help reduce the risk of hydrogen embrittlement; however, these measures only serve to lower the risk and cannot eliminate it entirely.

Anti-corrosion

What is the expected service life of the fastener, and under what environmental conditions will it be used? ASTM B633 defines “service conditions” as follows:

Usage Condition 1: Mild – Exposure to minimal condensation and minimal wear in indoor environments. For example: buttons, wire products, fasteners.

Condition 2: Moderate – Primarily exposed to dry indoor environments, but occasionally subject to condensation and abrasion. Examples include tools, zippers, drawer slides, and machine components.

Condition of Use 3: Severe—exposure to condensation, perspiration, infrequent wet rain, and cleaning agents. Examples include: pipe‑rack furniture, insect screens, door and window hardware, architectural hardware, military hardware components, washing machine parts, and bicycle components.

Condition 4: Extremely severe – exposure to harsh conditions, or frequent contact with humid environments, cleaning agents, and saline solutions, coupled with surface dents, scratches, or abrasion. Examples include sanitary ware and utility pole fittings.

Prevent injury during operation

Damage incurred during handling can be defined as scratches and abrasions sustained during transportation and/or installation, and it calls for careful attention to the surface treatment of fasteners. If not properly managed during handling and installation, softer or highly brittle surface treatments may enter the corrosion cycle at an earlier stage. Ensuring optimal adhesion of the surface treatment to the fastener is also a critical factor in maintaining superior corrosion‑resistance performance throughout service.

The criticality of connectivity

Can loose connections lead to assembly failure? What are the consequences of such failure? The effectiveness of both connection locking and retention hinges entirely on adequate clamping force. A well‑designed joint, combined with the proper clamping force, will never loosen. But how does this relate to the surface treatment of fasteners? In most assemblies, torque is used as a control parameter to ensure that repeated cyclic loading generates a predictable, consistent clamping force. Some surface treatments exhibit a specific coefficient of friction, while others do not. When sustained, repeatable clamping force is critical for the integrity of the joint, it is essential to employ surface coatings with a known coefficient of friction.

Functionality:

Fasteners with small diameters, as well as threaded fasteners featuring internal‑drive locking mechanisms, may be challenging to finish effectively. The type of surface treatment and the application method can result in residual coating material remaining in the threads and/or recesses. Please refer to the limitations and recommendations below.

The following guidelines provide information on the most common fastener surface treatments to help you make an informed choice:

Electro-galvanized (“commercial” galvanizing)

 Safety: High-strength fasteners carry a risk of hydrogen embrittlement.

 Corrosion protection: Mild to moderate corrosion protection

 Damage-resistant treatment: standard

 Critical connection: Friction is difficult to control

 Functionality: No issues with small‑size threads or slots.

 Cost: Low

 Availability: High

 Recommended applications: Any fasteners with mild to moderate corrosion‑resistance requirements, where the hardness is below HRC 38.

Electro-galvanized nickel

 Safety: The risk of hydrogen embrittlement is lower than with electroplated zinc, but it still exists.

 Corrosion resistance: Very high to extremely high corrosion resistance

 Damage-resistant treatment: Good

 Critical connection: Friction is difficult to control

 Functionality: No issues with small‑size threads or slots.

 Cost: High

 Availability: Low

 Recommended applications: Fasteners requiring corrosion resistance across all height ranges, up to extremely high altitudes, with a hardness below HRC38. With proper control and optimization of the baking process, this may represent the best alternative for surface treatment of small‑size, high‑strength fasteners up to M6 in size.

Mechanical zinc plating

 Safety: No risk of hydrogen embrittlement

 Corrosion protection: Mild to moderate corrosion protection

 Damage-resistant treatment: standard

 Critical connection: Friction is difficult to control

 Functional performance: Poor adhesion to grooves or holes—unsuitable for threaded components.

 Cost: Medium/High

 Availability: Medium/Low

 Recommended applications: Any high-strength washers and pins (with a hardness exceeding HRC38); generally not recommended for threaded fasteners.

Non-electrolytic zinc coating

 Safety: No risk of hydrogen embrittlement

 Corrosion resistance: Very high to extremely high corrosion resistance

 Damage-resistant treatment: standard

 Critical connection: Outstanding friction performance control

 Functionality: It is generally not recommended for use on threaded fasteners with a thread size of M8 or smaller, or on those employing an internal drive mechanism.

 Cost: Medium

 Availability: Medium

 Recommended applications: Large‑size fasteners (larger than M6) with extremely high corrosion‑resistance requirements, and/or connections that demand clamping force and where maintaining connection integrity is of critical importance.

Hot-dip galvanizing

 Safety: For high-strength fasteners, the risk of hydrogen embrittlement is low; however, due to the possibility of tempering hardness during machining, do not use on 12.9‑grade or alloy‑steel components.

 Corrosion resistance: Very high to extremely high corrosion resistance

 Damage-resistant treatment: Excellent

 Critical connection: Friction is difficult to control

 Functionality: It is generally not recommended for use with threaded fasteners smaller than M8 or those employing internal drive mechanisms. – After coating, the nut/internal thread must be re-tapped.

 Cost: Medium

 Availability: Medium

 Recommended application: For use in outdoor structural environments.

Epoxy electrophoretic coating

 Safety: No risk of hydrogen embrittlement

 Corrosion protection: Moderate to high corrosion protection

 Damage-resistant treatment: standard

 Critical connection: Outstanding friction performance control

 Functionality: Typically used on small-sized fasteners with in‑band drive, or on products with thread sizes of M4 and larger, where no issues arise.

 Cost: High

 Availability: Low

 Recommended applications: self-tapping locking screws or screws with a highly decorative black finish.

The final issue that was not addressed earlier is the presence of hexavalent chromium (Cr VI or Cr 6+) in fastener surface treatments. Many countries currently regulate the use of hexavalent chromium, a substance that was widely employed in past fastener surface‑finishing processes and can still be found in certain regions today. All of the aforementioned surface‑treatment methods are specified as “free of hexavalent chromium,” but it remains essential to verify whether this complies with the requirements of the intended application.

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