Every casting will corrode when it comes in contact with moisture, salt, and other aggressive environments. If and how fast a casting will rust will depend on the base material and the environment. If the casting is made from unprotected cast iron, carbon steel, or rustable aluminium it will rust in humid or salt rich environments, gaining oxidation. If the casting is made from stainless steel, or aluminium, or copper alloys it will have weaker and more corrosion susceptible protection. Regardless, all those will corrode protected castings in aggressive environments (highly concentrated chlorides, chemicals, or with close crevices) and cause pitting or localized corrosion.
To protect unprotected castings, there are different approaches. Each one of these has different cost and efficiency targets.
Painted or coated surfaces
Benefits: Having a paint or other surface coating will isolate air and moisture from the metal surface and will slow down corrosion significantly for an extended period of time and at a very low cost.
Considerations: Coatings must be kept intact. Scratches, chips, or otherwise failed areas of the coating will expose unprotected metal, and these will initiate corrosion. If the coating/material is damaged repair must be done quickly and coated areas need regular inspection.
Hot-dip Galvanizing vs. Electroplating
Hot-dip galvanizing is fantastic for outdoor or wet conditions. Immersing steel castings in molten zinc provides strong sacrificial protection along with a nice durable zinc coating. Zinc is a sacrificial anode which means it will corrode preferentially to the steel it is galvanizing along with creating a thick coating that will steel galvanizing.
Electroplating is also an option for adding zinc to a product for a beautiful finish, although it does not provide harsh outdoor protection. Corrosion protection is a big value with zinc plating, but it is not a permanent solution. It is a nice touch.
Nickel or Chrome Plating
Having coatings of nickel or chrome is a beautiful finish that provides protection against corrosion. These coatings will fully encapsulate the surface creating a hard, smooth, and an abrasion resistant layer along with retarding corrosion.
There are several more complex surfaces that also will add to the pricing of these coatings. In certain unfavorable conditions, the coatings will lose protection and become corroded.
Black Oxide or Phosphating
Black oxide and phosphating, along with several other treatments, provide a form of secondary protection and improved paint or coating adhesion for fastening or machined surfaces.
While these treatments are excellent for certain purposes, it is not the primary defense to the problem. Auxiliary protection works best along with other paints or seals.
Anodizing Aluminum
Anodizing is the best form of surface finishing. It adds to the surfaces protection of aluminum against a harsh environment and surfaces of hard oxide coating become intrinsically a part of the substrate.
Considerations: Aluminum alloys would yield the best results; the processes and sealing have an impact on the performance.
Protective coatings paired with cathodic protection systems.
In extreme environments, combined systems and cathodic protection (sacrificial anodes, impressed current) offer multilayered, complete corrosion control systems, especially for complex and underwater conditions.
To summarize, combining substrate casting protection with environmental considerations and budget is essential. For low-cost general-purpose scenarios, use paint; for outdoor steel components, prefer hot-dip galvanizing; for aluminum alloys, focus on anodic oxidation; for extreme environments, coatings combined with cathodic protection are vital. A well-balanced protection plan will greatly increase the service life of castings and minimize repair expenditures.