Any machined part taken directly off of a mill or lathe will have distinctive scratches or marks due to the milling or turning processes. While a rough surface may be acceptable for functional prototypes or parts hidden from sight, most components require a surface finish to complete their manufacturing. Not only does the right finish make the part more aesthetically pleasing, but it also improves the quality of the part with essential protection. At Plethora, we have a high level of expertise with surface finishes for metal parts and can help you choose the best finish for your precision components.

A Look at Surface Finishes for Metals

Treating a metal component with a surface finish will improve the life and performance of the part by protecting it in several ways:

  • Abrasion resistance
  • Improved aesthetics
  • Chemical resistance
  • Increase durability
  • Corrosion resistance
  • Electrical resistance

Surface finishes on metal will also reduce friction, improving the work and wear and thermal and electrical conductivity. At Plethora, our engineers understand which surface treatments will work best for your parts’ functional requirements and can help you choose between the various finishes.

Types of Finishes

Designers select surface finishes to improve a product or component, and the best treatment depends on the component’s function and the materials involved. Here are some of the surface finishes available from Plethora for you to choose from:

Anodizing

From parts used in aerospace to consumer electronics, anodizing creates a corrosion-resistant finish that is both durable and aesthetic. Used chiefly for aluminum, anodizing involves an electrochemical process that molecularly bonds the anodic film to the substrate. Anodizing offers a range of colors and gloss finishes that won’t wear off, and a cost-effective finish available in the following configurations:

  • Type I: Hard anodizing with a chromic acid-based chemical bath produces a thin, durable coat with a high level of resistance to corrosion. Coatings range from 0.00002 inch to 0.0001 inch (0.00127 mm to 0.00254 mm).
  • Type II: Sulfuric acid anodizing allows metals to absorb most colored dyes, thus often referred to as decorative anodizing. Thickness can go up to 0.001 inches (0.0254 mm).
  • Type III: Sulfuric acid “hard anodizing,” also called hardcoat, uses additional electrical current to produce the thickest anodized finish ranging in color from gray to bronze. This finish can achieve thicknesses up to 0.003 inches (0.0762 mm).

Alodine Coating

Alodine coatings use chrome phosphorus technology to create a surface chemical reaction on aluminum. In this process, a finishing technician will dip aluminum alloy parts into liquefied chrome phosphorus, where the resulting chemical reaction creates a protective film. Most aluminum alloys used for aviation parts involve alodine finishing to isolate, preserve and improve components, and ensure their conductivity.

Powder coating

 A worker in protective clothing powder-coating metal parts

Though most industrial surface finishes for metal come in liquid form, powder coatings offer an alternative, thicker coating. Powder coating allows more accessible additions of pigments, level agents, flow modifiers, curatives, and other additives that create visual appeal in addition to protecting the part. Machine shops apply the powder using an electrostatic spray deposition process which creates chemical reactions that bind the coatings to the component’s surface.

 

Passivation

Passivation uses a chemical process that treats the metal component by submerging it into a citric or nitric solution to remove particles such as iron that are susceptible to corrosion. The remaining elements of the alloy form an oxide layer when exposed to the air, creating a protective shield that prevents rust without changing the part’s physical appearance. Materials that use this metal finishing method include:

Electro Polish

In an electrochemical metal finishing process, electropolishing removes a thin layer from a part, typically made from stainless steel or comparable alloys, leaving a clean, shiny, and smooth finish. Machinists also refer to the process as anodic, electrochemical, or electrolytic polishing. It involves polishing and deburring fragile components or those with complex shapes. Like passivation, it uses similar chemical and non-mechanical processes, though electropolishing uses electrical current.

Blasting

Typically used to apply a uniform matte texture, blasting forces abrasive material at high speed against the surface. Also called bead blasting, it results in a clean, smooth texture, especially on soft metals.

A cold working blasting technique called shot peening helps improve a part’s resistance to fatigue, corrosion, and cracking. Another cold working blasting method, burnishing, also helps components resist metal fatigue.

Plating

Examples of different colored powder paint for metal parts

Also referred to as electroplating, machinists use this process to apply a coating of a variety of different metals:

  • Zinc plating: Zinc works admirably as a finish over steel or iron to prevent the metal from corroding. However, zinc plating does not work well for components exposed to temperatures greater than 500˚ F (260˚ C) or those in marine environments.
  • Tin plating: Quite standard as a coating due to its low cost, tin offers a soft and ductile covering that resists corrosion while allowing easy soldering. Often used in the electronics industry and for computer components, it provides an alternative to electroplating.
  • Copper plating: Copper delivers a variety of surface finishes for metal, producing dull, semi-bright, satin, polished, or gloss coatings. It has high electrical and thermal conductivity, magnetic repellence, antibacterial and ductile properties as a chemical element.