The variety and quantity of components in the automotive manufacturing industry are immense, with metal parts accounting for approximately 60% to 70% of the total. Most of these metal parts require surface treatment. Through surface treatment, the original characteristics and properties of the substrate material can be maintained while imparting new performance requirements to the part’s surface. Simultaneously, it can alter the surface condition and properties of the part, enhancing its physical and mechanical performance. Currently, widely applied surface treatment technologies in the field of automotive component manufacturing can be divided into two main categories: chemical treatment methods, which include electroplating, electrophoresis, and passivation; and mechanical treatment methods, which include shot blasting, sandblasting, and spray coating. Different types of surface treatment technologies vary in their methods, functions, and purposes, and their process flows are entirely different. Consequently, the test plans prepared for automotive part validation projects also vary. If a test plan is formulated unreasonably or incompletely, it will directly impact the quality and progress of new part development projects.
Functions of Surface Treatment
Surface treatment is a process method that forms a surface layer on the substrate material through various physical or chemical methods, possessing mechanical, physical, and chemical properties different from those of the substrate. The primary purposes of surface treatment for CNC metal parts are as follows:
Decoration
Surface treatment can make the surface bright and enhance the product’s aesthetic appeal. For example, exposed metal parts such as car logos, bumpers, door handles, window frames, and wheel hubs require a glossy and beautiful appearance. After chrome or zinc plating, the aesthetic appeal of the product is greatly improved, making it more popular among consumers.
Altering Surface Performance
It improves the surface properties of parts in terms of corrosion resistance, wear resistance, fatigue resistance, and oxidation resistance. For example, moving parts in an engine that must withstand impact loads, such as pistons, piston rings, and connecting rods, can have their surface hardness, wear resistance, and fatigue resistance improved through surface carburizing or nitriding while the core of the part maintains good plasticity and toughness. Fasteners such as bolts, screws, nuts, and washers are typically subjected to surface treatments like zinc plating, nickel plating, or black oxidation after processing to improve corrosion and oxidation resistance.
Altering Surface Finish and Flatness
The surfaces of cast and forged blanks are usually rough. Methods such as shot blasting and polishing can be used to remove burrs, oxide scale, and rust from the surface, thereby improving the flatness of the part’s surface.
Altering Thermal Conductivity or Insulation Properties
For components that require heat transfer or dissipation, high-thermal-conductivity materials can be used as surface fillers to improve conductivity. Alternatively, insulation materials can be used for surface treatment to enhance the thermal insulation properties of the part.
Improving Surface Conductivity or Insulation
Electrical components or parts requiring welding must have good electrical conductivity. This can be achieved by adding elements with high conductivity, such as copper, aluminum, or silver, for alloying, or by electroplating a layer of high-conductivity metal on the substrate surface. For metal materials requiring surface insulation, this can be achieved by spraying insulating paint or adhering insulating materials such as plastic films, rubber, or resin.
Improving Surface Adhesion
For components that require subsequent spraying or painting, surface treatment methods such as sandblasting, phosphating, or electrochemical treatment are usually employed to improve the surface flatness of the base material, thereby enhancing the adhesion between the coating and the substrate.
5 Main Surface Treatment Methods
The main processing technologies for common automotive metal parts include machining, stamping, die casting, forging, and powder metallurgy. Metal parts produced by different processes have entirely different physical and mechanical properties. Since the purposes of surface treatment also differ, the applicable surface treatment methods and corresponding part validation test plans vary accordingly. The most common surface treatment methods for automotive metal parts include electroplating, shot blasting, sand blasting, shot peening, and spray coating.
Electroplating
Electroplating is a surface treatment method where the metal substrate is immersed in a liquid solution containing compounds of the coating metal. When the solution is energized, the coating metal precipitates and deposits onto the substrate material. This is a common method for body stamping panels and metal fasteners like bolts, nuts, and washers, requiring the substrate to have good conductivity to improve corrosion resistance and aesthetics. Depending on the purpose, different coating materials can be selected, such as zinc, chromium, copper, nickel, or tin, each with its own advantages and disadvantages.
Among these, zinc plating accounts for 40% to 50% of electroplating, making it the most widely used method. The corrosion resistance of zinc-plated parts is directly related to the coating thickness. When the thickness is 5–8 μm, the Neutral Salt Spray (NSS) test requirement is typically 48 hours without white rust and 72 hours without red rust; for 8–13 μm, the requirement is 72 hours without white rust and 120 hours without red rust. During zinc plating, hydrogen is evolved at the cathode, and hydrogen atoms penetrating the substrate can cause hydrogen embrittlement. Embrittled parts carry a risk of fracture during subsequent use, which could lead to serious accidents and affect product safety. Therefore, fasteners with a strength grade higher than 10.9 must undergo a de-hydrogenation (baking) process after plating and before passivation.
Shot Blasting
Shot blasting relies on centrifugal force to hurl projectiles of various materials (stainless steel shot, cast steel shot, etc.) with diameters of 0.2–3.0 mm onto the part surface. This removes grease, dirt, oxide scale, corrosion, oxides, and other impurities. It also removes machining burrs, eliminates internal stress, reduces deformation after heat treatment, and improves surface wear resistance and pressure-bearing capacity. Unlike polishing, which smooths the surface, shot blasting makes the surface rougher, providing a good base for subsequent spraying or painting, which improves coating adhesion and part longevity.
Parts treated by shot blasting require visual inspection, cleanliness level testing, surface roughness testing, and surface coverage testing. Visual inspection ensures the surface is free of rust spots, scale, and dirt. Quality levels are categorized into four grades (from “most thorough” to “non-thorough”) based on the proportion of shadows and color differences caused by incomplete cleaning.
Sand blasting
Sandblasting uses a high-speed jet formed by compressed air to spray abrasives like iron sand or corundum onto the part surface. This removes impurities and increases surface cleanliness, thereby altering surface roughness. Sandblasting can also modify the appearance, shape, hardness, and wear resistance of the part, improving surface adhesion. Because sandblasting provides superior cleaning effects, it is preferred when surface treatment requirements are high.
Shot Peening
Shot peening uses compressed air as the power and frictional force to remove oxides, rust, and dirt using metal shots with diameters of 0.2–2.5 mm. Cast and forged parts often have residual molding sand, rust, or oxide scale that can affect appearance, dimensions, or performance. Therefore, shot peening is the most common surface treatment for complex-shaped cast or forged blanks. Its effects and test items are fundamentally like shot blasting.
Spray Coating
Spray coating typically uses compressed air to atomize liquid paint into uniform, tiny droplets through a spray gun, which are then projected onto the part surface. Methods include air spraying and electrostatic spraying; the latter offers higher paint utilization but requires a conductive substrate.
Coated parts must undergo inspection for appearance, coating thickness, surface hardness, adhesion, corrosion resistance, and environmental resistance. Common defects include seeding, sagging, orange peel, blushing, and wrinkling. Surface hardness is often tested using the Pencil Hardness Test (HB grade), ensuring that only slight scratches are permitted without breaking the film or exposing the substrate.
Adhesion testing usually refers to the ISO 2409 Cross-Cut Test, where a grid of 1 mm x 1 mm squares is cut into the coating. 3M tape is applied and then peeled off at a 45° angle after 1 minute to determine the adhesion grade based on the area of coating detached. Additionally, depending on the application, validation may include high-low temperature cycling, solvent resistance, and friction resistance tests.
Conclusion
Due to varying processing technologies and usage requirements, different surface treatment methods and corresponding validation plans must be selected for automotive metal parts. Only by formulating reasonable, accurate, and complete test plans can the quality of surface treatments be ensured to meet customer requirements. As components account for 60% to 70% of the total vehicle cost, manufacturing enterprises are continuously researching surface treatment technologies to reduce costs while moving toward more energy-efficient, environmentally friendly, and efficient solutions.
