Customers ask me, “Why do white spots happen on anodized parts?”
The quick answer is those white spots are corrosive cells found beneath the anodized coating. The unfinished metal was probably not prepared properly and most likely, the coating applied was not thick enough. When this happens, moisture penetrates the coating and gets locked under the finish, causing oxidation to occur.
When this happens, the best way to deal with it is to remove all the corrosion mechanically, then clean the surface with a stabilizing solution before applying a new anodic coating.
This defect can also happen on electro-colored finishes using nickel or cobalt-based coloring electrolytes. This shouldn’t be mixed up with the spalling or anodic film detachment that can sometimes occur on some anodized and dyed strong alloys like Al-Mg-Zn. This phenomenon is called blistering. Reprocessing the samples can eliminate this type of defect as long as measures are taken to extract the caustic mist.
As I’ve stated in previous blogs about white spots, there are several ways to prevent white spots from happening. They are:
1) For aluminum alloys – The aluminum alloy EN AW 2024-T3511 often has problems with white spots. It’s because copper and magnesium, as well as iron are elements contained in this type of alloy. Galvanic corrosion can occur in these alloying elements due to its microstructure. It’s been well documented that the EN AW 2024 aluminum-copper alloy creates inhomogeneous oxide layers in the areas of the intermetallic phase, CuAl2 and other copper containing phases. This alloy is known to be both sensitive to (See below graphic a & b) galvanic corrosion, but it’s also more prone to (c) pitting corrosion.
Image c) shows white spots occurring from galvanic corrosion. You’ll note that it’s often “tailed” (see the streaky white areas) without any specific direction. This indicates that the spots are not dependent on agitation of the solution or other directional issues.
2) During the pre-treatment process – For inhomogeneous microstructured alloys, it’s critical to be sure that chloride is NOT used in any of the chemical pretreatment steps. This also means ensuring that the rinse water tanks used are checked regularly since municipal water supplies are often used by metal finishers and this usually contains chloride.
Changing the incoming water could be detrimental for production, but chlorides are guaranteed to create white spots on EN AW 2024 alloys. Additionally, shortening the process time or eliminating the pretreatment steps by using a very high pH (alkaline) or very low pH (acidic), can reduce these corroded spots from appearing.
3) The anodizing process – Whenever EN AW 2024 is anodized, an inhomogeneous anodic layer is formed with varying layers of thickness and cracks.
These cracks cause weak spots in the oxide layer, making the surface not only less corrosion-resistant, but also receptive to a small galvanic current to run through it. Weak spots are more anodic than the rest of the coherent oxide layer, thus creating small galvanic cells and leading to a corrosion attack on the black surface, appearing as white spots. This is often first observed following the coloring or sealing process.
Similar to the pre-treatment step, chlorides found in anodizing tanks are often caused by sulfuric acid or additives and this can cause white spots. The same can be said about chlorides found in the dye or sealing additives.
Again, to prevent white spots from happening, the rinse water needs to be checked for chloride since the surface is more prone to galvanic attacks during the anodizing step. This happens because the formed oxide layer is non-conductive, so if a current is used, there are small areas that can be attacked by chloride. This can lead to severe galvanic corrosion attack as seen in Figure 4 below.
Fig. 4: SEM image of a black anodized surface seen from the top with a lot of galvanic attacks.
By using different anodizing parameters like ramping, pulsing, or changing the concentration of sulfuric acid, a homogeneous and coherent oxide layer on these alloys can be created.
4) Titanium racks – Racks made of titanium can sometimes increase the risk of galvanic corrosion because the titanium itself can contain a small current from the anodizing process.
Aerospace Metals LLC is an industry leader with 30+ years of professional experience. Owner Paul Fredericks and his team of professionals can help you achieve your metal finishing goals without worrying about the white spot issue. That’s because Paul’s team of metal finishing professionals have cutting edge technology and equipment, as well as best practices to prevent pitting and white spots from occurring. Aerospace Metals has the experience you need to produce high-quality metal products to exacting specifications.
Visit the Aerospace Metals website at High-Quality Metal Finishing Services (aerospacemetalsllc.com) to get a free, no obligation quote today!
