The “ VW TL 244 Zinc/nickel alloy coatings - Surface protection requirements” standard, developed by the Volkswagen automotive company, describes the requirements for electrolytically deposited and post-treated zinc/nickel alloy coatings that do not contain hexavalent chromium, on ferrous materials and steel parts with tensile strengths of 1,000 MPa and higher, as specified in the “r” code letter of the VW 13750 standard. This standard also defines application limits and exceptions for tensile strength values exceeding 1,000 MPa.
Alloy coatings defined in this standard shall not be used for steel components with a tensile strength exceeding 1 200 MPa and a surface hardness exceeding 370 HV. Exceptions may be made for components with a tensile strength between 1 000 MPa and 1 200 MPa. However, in these cases, heat treatment in accordance with DIN EN ISO 4042 is required.
Used as heavy-duty corrosion protection (protection class 6), these coatings are particularly suitable for components and systems of threaded connections, where an increased corrosion load as well as a temperature load of up to 150 degrees (engine compartment and brake area) are expected. The silver-coloured coatings Ofl-r642, Ofl-r643 and Ofl-r645 are particularly suitable for electrically conductive connections (ground connections). In particular, these coatings are suitable for connection elements with drive recesses to prevent blocking of force application points. Due to their electrochemical compatibility, they are also suitable for use in contact with aluminium materials, but not for use in direct contact with magnesium materials.
Depending on the protection class, chemical composition, structure and appearance, it is decided to use one of the surface protection types defined in the table in the standard for the coating system according to this standard. The definitions of these surface protection types are included in the VW 13750 standard “Definition”.
The surface roughness of zinc/nickel coated parts depends, among other things, on the properties of the zinc/nickel electrolyte or the surface quality of the component. The surface roughness difference between the uncoated and coated component should be 5 µm or less. Significant nodulation on the zinc/nickel surface in conjunction with severe cracking should be avoided as this adversely affects the properties of the coating (e.g. flaking, inadequate corrosion protection, undesirable welding of components due to friction during assembly). A homogeneous structure should be ensured for the zinc/nickel coating, showing fine cracks from the surface to the substrate.
The requirements of the VW TL 244 standard are explained under the following headings:
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