Surface Treatment of Fasteners

General

Surface finishing methods are applied to "bare" fasteners that show a tendency toward "rusting" due to hydrogen embrittlement (C). Surface finishing is performed to coat the fasteners with a protective layer. In this way, significantly better or worse protection is provided depending on the surface type. Surface finishing only slows corrosion. If fasteners do not have lower hardness, only fasteners made from stainless steel (for example, A2 steel) must be used to prevent corrosion.

Metal Coatings

In metal coatings, zinc or zinc alloy layers typically result from galvanizing. Metal coatings are less resistant to corrosion than inorganic coatings. However, durability effects can be increased by sealing with organic layers. In coatings where the galvanizing method is applied, there is a risk of hydrogen embrittlement due to hydrogen absorption in the pre-treatment (for example, acid pickling) and zinc coating process. To eliminate this risk, such products must be subjected to additional heat treatment. For more detailed information on this, you can consult the standard DIN EN ISO 4042 "Galvanized Coating of Fasteners". Embrittlement caused by hydrogen is sudden, always delayed in duration, and is a fracture without deformation.

There are two types of fracture:

• Production-related embrittlement fracture, caused by galvanizing surface finishing or pre-treatment. • Service-related embrittlement fracture, caused by corrosion, or more precisely, hydrogen from chlorides of a previous corrosion attack.

How can hydrogen-based embrittlement be reduced in high-strength and hardened fasteners?

• Avoid and reduce hydrogen sources (for example, water vapor), • Manufacture fasteners from stainless steel (for example, A2 type steel), • Prevent stress concentration, • Do not apply acid treatment, • Subsequent industrial processes, • Subsequently applied heat treatment (tempering) • Application of organic and/or inorganic coatings • Do not apply galvanizing to high-strength or hardened parts

Inorganic Coatings

These layer formation types consist of zinc and aluminum lamellar mixtures applied by burning at high temperature. These methods are gaining increasing importance due to their property of providing excessive corrosion resistance. Hydrogen-related embrittlement does not occur in inorganic coatings because no surface cleaning process is performed in this method. Details regarding this type of coating are contained in the standard DIN EN ISO 10683 "Zinc lamellar coatings applied without electrolytes". These are in part as follows: • Infusion of non-metal parts, • Infusion of metals, • Infusion of metal and non-metal parts, • Effusion of metal and non-metal parts.

Organic Coatings

This form of coating offers only partially, almost unlimited color selection coating option. Its basis is founded on natural products. These coatings have in part very high permanent heat resistance and do not conduct electricity. There is no risk of hydrogen embrittlement as long as the material surface is not cleaned with acid. Once the layer on the surface is removed, cathodic corrosion protection no longer exists. At initial delivery, corrosion resistance is very high.

Electroplated Zinc Coating (Galvanizing) General, Layer Thicknesses

Brief description example for desired galvanizing surface finishing ~ Should not be confused with description of high-quality steels (for example A2-70) ~ Abbreviation marks for galvanized surfaces: A2F 1) Coating material identification letter A = Zinc (Zn) 2) Layer thickness code in μm 2 = 5 μm 3) Brightness level and subsequent treatment code F = matte, bluish color

For tests, layer thickness at the measurement point applies

  Thread tolerances are particularly valid before the galvanizing coating process. With the coating process, the zero line of the external thread or nut thread must not fall below the value. The coated external thread can be between the upper measurement point of the tolerance area and the zero line. Regarding threadability, the layer thickness of parts with thread areas is logically limited to the general tolerance area 6g/gH.  

Layer thicknesses

Table 1: Maximum layer thicknesses for externally threaded fasteners

Table 2: Forcing and suitable zinc layer thicknesses

Contact Corrosion

Table 3: Contact corrosion in galvanized zinc coating

   

Annual Wear Values

Table 4: Annual worn layer thickness of zinc in flat surface corrosion

Hot-Dip Galvanizing

The minimum layer thickness required by norm at the 40mm measurement point requires the fastener thread to be of a smaller extent; this lower value is generally the external thread. The galvanized external thread must not exceed the zero line. The external thread cannot be cut subsequently. In HV connections, a tolerance value is given to the DIN 6914/6915 nut (= Z/X/AZ/AX) – As a result, the galvanized external thread is above the zero line. Nut threads are subsequently cut into the galvanized blank parts. Zinc coating of the external thread creates protection against corrosion (cathodic protection). Galvanized fasteners will show lower strength than non-galvanized fasteners (due to less edge coating in the thread) Consequently, test/fracture strength decreases. In assembly of galvanized fasteners and nuts, particularly in increased thread lubrication, friction and torque moment values may vary.

Contact corrosion in zinc-containing raw materials (shown in yellow)

Annual wear values in zinc use, flat surface corrosion

    İzzet Aydın General Director Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti.    

Sources • DIN EN ISO 4042 "Galvanische Überzüge für Verbindungselemente". • DIN EN ISO 10683 unter dem Begriff "Nichtelektrolytisch aufgebrachte Zinklamellenüberzüge". • ISO 6157-1 (DIN 267-1 9, Abs. 2.7). • ISO 4042 Abs. 6 / Anhang A / ISO 15330. • ISO 61 57-2 / EN 493 (DIN 267-19, Abs. 2.7) EN 1403, 12329 Beratungsstelle "FEUERVERZINKEN". • ISO 10684 (DIN 267-10). • DIN 18800-7/EN V 1090-1. • Schrauben Lexikon. • Tabellenbuch Verbindungselemente.