WHY DO WE NEED SURFACE TREATMENT?
The primary purpose of surface treatment is to give the fasteners protection against corrosion. Another vital function is friction control which purpose is to ensure that users can correctly assemble the fasteners. The color can be controlled to some extent by surface treatment to make the appearance of fasteners match with the rest of the application or to differentiate the fasteners in an assembly line.
Corrosion is a natural process. In dry air, all metals form a thin oxide by a chemical reaction with oxygen in the air. With increased humidity in the air, another corrosion mechanism takes place called electrochemical corrosion, which is a reaction between metal, oxygen and moister covering the metal. For low alloy steel, this reaction results in different iron oxides.
The electrochemical corrosion is a redox reaction which consists of two half-cells called oxidation and reduction. During oxidation, iron gives away electrons, resulting in iron ions and electrons. During reduction the released electrodes, water, and oxygen reacts, forming hydroxide. The total reaction is the redox reaction, saying that iron ions and hydroxide ions react, resulting in iron oxide.
How severe the corrosion attack is, depends on the surroundings. Carbon steel exposed to a humidity of at least 60% forms a uniform corrosion layer covering the whole exposed part. The attack is not harmful in room temperature. However, the corrosion rate may increase dramatically with increased temperature, low pH and in the presence of salt.
The corrosion rate can also increase if two dissimilar metals, with different electrode potential, come in contact while surrounded by an electrolyte. This results in a difference in electron potential, resulting in a stream of electrons from the less reactive metal to the most reactive metal. The less reactive metal will suffer from severe corrosion while the other one remains intact.
The friction determines the clamping force achieved with a torque assembly strategy. If the friction is too high in the bolted joint, the torque applied may not be sufficient to create enough clamping force in the joint. If friction is too low, there may be a risk of breaking the bolt during assembly. To control the friction of mass-produced fasteners, surface treatments which include a lubricant, are used. In case of thread forming bolts, a layer of wax can be applied on top of the surface treatment.
If the fastener is visible to the end user, it is vital that it has a finish that matches the surrounding components. Most common colors are black or metal-colored, i.e., a silvery finish. Different colors on different bolts can also facilitate assembly. An example of this can be that bolts of the same size, but with different tensile strengths have different colors.
The surface treatment can be seen as encapsulating the metal in a protective shell. In this way, the metal will not come into contact with oxygen and moisture, thus inhibiting the corrosion process. In this protection, friction control and appearance are also added. The protection is usually cathodic, meaning that it consists of a metal with a lower electron potential and will, therefore, be consumed before the base material in a corrosive environment.
Applying surface treatment is a complicated process which can involve up to 50 different steps. However, the process can be divided into three main stages:
- Pre-treatment: Cleaning and activation of the surface which is to be coated.
- Treatment: Application of base coat for corrosion protection.
- After-treatment: Application of top-coat for extra protection, friction control, and appearance.
Although surface treatment has many essential features and can consist of several layers, it is surprisingly thin; only a few μm – Thinner than a human hair.
SURFACE TREATMENT TYPES
There are three main groups of surface treatment types:
With electrolytic surface treatment, a new metal layer is created on top of the substrate by electrolysis. Zinc and zinc-nickel alloys are the most common coatings, which both work as sacrificial metal, i.e., it corrodes before the underlying metal does.
On top of the protecting metal-layer, a passivating layer is applied. This layer consists of chromium which has excellent corrosion resistance. For Zink Nickle, it is possible to choose a black passivate for a black appearance or a transparent passivate for steel appearance.
Next layer is the top coat. There are several different variations of top coats to meet different friction requirements. However, it also adds extra protection to the underlying base coat. Electrolytic surface treatment requires a perfect pretreatment. If the bolts are not clean enough the formation of the new metal layer cannot start correctly, which may result in reduced adhesion or no coating at all. The cleaning process involves many steps, e.g., degreasing and pickling.
Zinc flakes are the most common flake coating for bolts. It consists of zinc and aluminum in combination with an organic binder. The coating is applied by dipping the parts in the coating substances. Any excessive zinc flake solution is spun off in a centrifuge. The coating is then heated to about 300°C whereby adhesion to the base material and the bond between the zinc and aluminum flakes occurs. The most common color is silver gray. It can also be combined with transparent or black topcoat with friction controlling properties.
Flake coatings are like a paint color, i.e., no metal layer is built upon the base material as for electrolytic surface treatment. Therefore, there are not the same requirements on the pre-treatment. For flake systems, washing and sandblasting is enough. Also, since the coating contains an organic binder, it is not conductive.
Zinc phosphate coating is a conversion coating in which a dilute solution of phosphoric acid and phosphate salts is applied via immersion. The solution chemically reacts with the surface of the coated component, forming a layer of crystalline zinc phosphate. Top coatings are not used for phosphate bolts. Instead, they are after-treated with oil for friction properties and extra corrosion protection.
Like for electrolytic surface treatment, the surface needs to be very clean to have a proper crystal growth. Therefore, the pretreatment also includes a pickling step to ensure a clean metal surface before the phosphating.
EVALUATION OF SURFACE TREATMENTS
The surface treatment is evaluated in several ways. A friction test is performed to confirm that the surface treatment provides the friction specified. Visual inspection and microscopy examinations are performed to ensure that the surface treatment has the right structure and correct color. A cycling test often determines the corrosion resistance in a corrosion chamber.
The test parameters measured are:
- Time to white rust (where the zinc- or zinc-containing base coat starts to corrode).
- Time to red rust (when the base material starts to corrode).
SELECTING SURFACE TREATMENT
Many factors are involved in the choice of surface treatment. Better corrosion protection generally cost more, and it is essential to evaluate the environment where a specific bolt is located. In mild environments, e.g., inside a car, there is no need for a more expensive high-end corrosion protection. Another critical aspect to take into account is conductivity.
Some coatings are not conductive and will therefore not be an option for grounded joints. Hydrogen embrittlement is also an essential factor when it comes to high strength bolts. High strength steel may become very brittle when exposed to hydrogen formation. Coatings processes that generate hydrogen may therefore not be an option for high strength bolts.
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