Surface treatment of fasteners – Electroplating
I. Electroplating Codes:
The suffix codes for surface treatment represent the type of electroplating on their surfaces.
We have compiled a detailed list of some common surface treatment methods used in fasteners. As shown in the table below:
| Surface Treatment | FINISH | Code |
| Zinc Plating | ZINC CLEAR | ZI |
| ZINC BLUE | ZU | |
| ZINC YELLOW | ZC | |
| ZINC BLACK | ZB | |
| Nickel Plating | NICKEL FLASH | NI |
| Chrome Plating | CHROME FLASH | CR |
| Tin Plating | TIN FLASH | ET |
| Natural Finish | NATURAL | NL |
| Oxidation | NATURAL ANODIZE | NA |
| BLACK ANODIZE | BL |
II. Regarding Electroplating:
Electroplating is a process of coating a layer of metal on fasteners to change their surface properties, making them less prone to oxidation and corrosion. The metal used for the coating is usually a corrosion-resistant one.
Electroplating not only enhances the corrosion resistance of fasteners but also increases their hardness to prevent wear, improves electrical conductivity and heat resistance, and makes the surface smoother and more aesthetically pleasing.
Electroplating is a technique that uses electrolysis to deposit a layer of metal film on the surface of metal or other material parts.
Here is a brief introduction to some commonly used coatings for fasteners.
1. Electro-galvanizing
Electro-galvanizing is the most commonly used coating for fasteners. It has a good appearance and is relatively inexpensive. It comes in various colors such as white zinc, blue zinc, color zinc, and black zinc. Compared with other metal coatings, zinc is a relatively inexpensive and easy-to-electroplate metal.
However, its corrosion resistance is average. In the neutral salt spray test of electro-galvanized fasteners, it can last for 72 hours. Of course, there are also special sealing agents that can extend the neutral salt spray test to over 200 hours, but the price is relatively high, about 5 to 8 times that of ordinary galvanizing.
2. Electroplated Nickel
Fasteners electroplated with nickel are typically used in places where both high corrosion resistance and good electrical conductivity are required. The electroplated nickel layer has high stability in the air. Due to the strong passivation ability of the metal nickel, a very thin passivation film can be rapidly formed on the surface, which can resist the corrosion of the atmosphere, alkali, and certain acids.
Electroplated nickel has excellent polishing performance. After polishing, its luster can be maintained for a long time, and the hardness of the nickel plating layer is relatively high, which can improve the wear resistance of the fasteners.
3. Oxidation
Oxidation blackening + oil coating is a very popular coating for industrial fasteners because it is the cheapest and looks good before the oil runs out. In the presence of oil, its neutral salt spray test can only last for 3 to 5 hours.
The torque – preload consistency of blackened fasteners is also very poor. If it needs to be improved, grease can be applied to the internal and external threads before assembly and then screwed together.
4. Electroplating Chromium
Chromium plating on fasteners is generally used for decorative purposes. The chromium coating is very stable in the atmosphere, does not easily discolor or lose its luster, and has high hardness and good wear resistance.
Good electroplated chromium fasteners are as expensive as stainless steel. Only when the strength of stainless steel is insufficient, are electroplated chromium fasteners used as a substitute. Therefore, they are rarely used in industrial fields with high requirements for corrosion resistance.
To prevent corrosion, copper and nickel plating should be done first before chromium plating. The chromium coating can withstand a temperature of 650°C, but like electroplated zinc, it has the problem of hydrogen embrittlement.
III. Electroplating Standards and Quality Inspection:
National standard for surface treatment of fasteners GB/T5267.1-2002, the standard for electroplating layer of threaded fasteners, includes two standards: GB/T5267.1-2002 electroplating layer of fasteners and GB/T5267.2-2002 non-electrolytic zinc flake coating of fasteners.
This standard is equivalent to the international standard ISO4042-1999, the standard for electroplating layer of threaded fasteners.
The main purpose of surface treatment of fasteners is to improve their anti-corrosion ability, thereby increasing the reliability and adaptability of their use. The main measurement standards are corrosion resistance and appearance.
The quality of the electroplating layer of fasteners is mainly judged from the following aspects:
1. Appearance inspection
The surface of the fastener should be smooth, with good gloss and no missed plating areas. There should be no dirt, pores, pinholes, peeling, burnt plating layer, dullness, peeling, skinning, obvious streaks, black plating residue, loose, cracked, and fallen passivation film, or severe passivation marks.
2. Plating thickness
The thickness of the plating layer of the fastener is directly related to its corrosion resistance in the atmosphere. However, if it is too thick, it may cause thread interference during installation. It is recommended that the general plating thickness be 4 to 12 μm.
The average thickness of the hot-dip galvanized standard is 54 μm (for nominal diameter ≤ 3/8, it is 43 μm), and the minimum thickness is 43 μm (for nominal diameter ≤ 3/8, it is 37 μm).
3. Plating distribution
Different deposition methods result in different aggregation patterns of the plating layer on the surface of the fastener. During electroplating, the plating metal is not evenly deposited on the outer edge, and thicker plating is obtained at the corners.
In the threaded part of the fastener, the thickest plating is located at the top of the thread, gradually thinning along the side of the thread, and the thinnest plating is at the bottom of the thread.
In contrast, in hot-dip galvanizing, the thicker plating is deposited at the inner corners and the bottom of the thread. The deposition tendency of the plating metal in mechanical plating is the same as that in hot-dip plating, but it is smoother and more uniform in thickness over the entire surface.
4. Hydrogen embrittlement
During the processing and treatment of fasteners, especially during the acid washing and alkali washing before plating and the subsequent electroplating process, hydrogen atoms are absorbed on the surface. During the deposition process, hydrogen is generated.
When the fastener is tightened, hydrogen migrates to the area with the highest stress, causing the pressure to increase beyond its strength and resulting in tiny surface cracks. Hydrogen then penetrates into the newly formed cracks. This pressure-crack-penetration cycle continues until the fastener breaks. This usually occurs within a few hours after the first application of stress.
To eliminate the threat of hydrogen embrittlement, the fastener should be heated and baked within 3 hours after plating to allow hydrogen to escape from the plating layer. This is typically done at a temperature of around 200°C, and the treatment time is determined based on the required tensile strength.
Since mechanical galvanizing is non-electrolytic, it effectively eliminates the threat of hydrogen embrittlement. Therefore, hydrogen embrittlement rarely occurs in hot-dip galvanized fasteners.
IV. Heat treatment of fasteners:
Heat treatment is a process in which fasteners are heated, held at a certain temperature, and then cooled to change their internal structure and obtain the desired properties, microstructure, and structure.
Annealing, normalizing, quenching, and tempering are the “four fires” of heat treatment. Among them, quenching and tempering are closely related and are often used together, and neither can be missing.
- Annealing involves heating the workpiece to an appropriate temperature, holding it for a certain period of time, and then slowly cooling it to achieve or approach a balanced state of the internal structure, releasing the internal stress generated in the previous process, and obtaining good process performance and use performance to prepare for subsequent quenching.
- Normalizing involves heating the workpiece to an appropriate temperature and then cooling it in the air. The effect of normalizing is similar to that of annealing, but it results in a finer microstructure. It is often used to improve the cutting performance of materials and can be used as the final heat treatment for some parts with lower requirements.
- Quenching involves heating the workpiece to a certain temperature, holding it for a period of time, and then rapidly cooling it in water, oil, or other quenching media such as inorganic salt solutions or organic water solutions.
- Tempering is the process of heating the quenched workpiece to a suitable temperature above room temperature but below 650°C for a relatively long time and then cooling it. This process reduces the brittleness of the quenched workpiece.
The four types of heat treatment processes have evolved into different techniques based on variations in heating temperatures and cooling methods. To achieve a certain level of strength and toughness, the process that combines quenching and high-temperature tempering is called normalizing.
If you are looking for quality fasteners, please visit us here https://hktl-fastener.com/.
