Having trouble with spotting defects when anodizing? Taj Patel of Techevon LLC offers a helpful overview of the various causes of white spots and potential solutions. #basics #management #asktheexpert
Q: What causes white spots when anodizing and how can I prevent them? A. White spotting/staining is a common phenomenon, however, understanding the cause of these surface defects can be challenging. The primary source for this confusion is that white spotting is not caused by the same issue all the time and in many cases, it could be a combination of multiple factors that result in the surface defect. Furthermore, some defects are related to the actual anodic coating, whereas others could simply be on the surface of the coating. In the list below, we have summarized the key properties of the anodizing process that can lead to white spots or other surface defects: Balcony Hand Railing
This graphic shows the multitude of different process tanks and areas that could result in surface defects that look like white spots.
Sources of spotting/staining include the following:
Solving the problem starts with understanding the alloy type. Potential solutions include managing rinsing (reducing immersion time, improving water quality), using sacrificial anodes (magnesium bars), chloride management (determining source and containing and reviewing tooling (Al vs Ti).
Machining quality and part profile can also play a role in spotting defects. Sources of spotting/staining can include sharp edges on parts, contaminated cutting fluid and poor material handling procedures.
Sources of spotting/staining pertaining to pretreatment include threaded blind holes (the smaller the hole the greater the problem) and poor rinsing, which contribute to the entrapment of process chemicals.
Potential solutions include eliminating pretreat (if possible), physically rinsing every hole on a load, and the use of special rinse aids in the tank.
When an anodizing load comes out of the tank at the same time as an etch load, caustic fumes can deposit on the anodized surface, particularly if air flows from the etch tank. Etch fume staining occurs when etch dries on the part prior to rinsing and usually results in spotting on the top of the load instead of the bottom.
In many cases, surface defects may exist on aluminum which can become more visible after anodizing, resulting in “white spots.” Below is an overview of potential surface defects to be watchful for.
Galvanic pitting occurs when dissimilar metals are electrically connected and the less noble of the metals corrodes. Particularly at sites where there are distinct breaks of the alloy constituents in the anodic coating. Galvanic activity can happen in several different process tanks and is random across a part which makes it difficult to identify. Several factors can cause galvanic activity:
Atmospheric or fallout corrosion can occur on incoming metal as a result of poor material handling. Fallout corrosion typically occurs in specific areas of the part (outside surface) instead of the entire part. There are several factors that contribute to fallout corrosion, including contaminated cutting fluid (i.e., algae), and moisture in part storage areas.
The two primary water contaminants to be aware of are chlorides and algae. Chloride levels can vary significantly depending on the source of water; therefore, using DI water for all tanks after the pretreatment will significantly reduce the likelihood of chlorides. Additionally, chemistry should be checked for chlorides.Contact your vendor to understand what chlorides if any could be in the following products:
Algae is typically linked to dyes because many dyes are consumed by algae. Therefore, dye tanks should be regularly dosed with algaecides, particularly in the summer months. Furthermore, when storing dyes for extended periods of time, be sure to follow supplier guidelines on storage. In many cases, liquid dyes are more susceptible to algae contamination.
Caustic fume staining occurs when parts are removed from the etch tank and are exposed to etch fumes for extended periods or when the etch dries on the part without immediate rinsing. There are several factors that can contribute to caustic fume staining/drying:
Bleed out occurs when hot etch solutions cause threaded holes to expand and trap caustic, which carries over to till the next hot process tank, which is the dye tank. When the etch “bleeds out” it creates white stains that do not take color. There are several factors that can contribute to bleed out:
Precipitates and other solids residue on a part are most often seen during the final stages of the anodizing process, such as dyeing or sealing. There are several factors that can cause precipitates or other residue:
Nickel hydroxide formation can be linked to a high pH (>6.0). When this happens, the nickel hydroxide precipitate forms and cannot go back into the solution.
For more information, including images of these defects, contact Tej Patel via email Tej.Patel@techevon.com.
Tej Patel is the director of Techevon LLC. Visit techevon.com. In addition, read Patel’s award-winning paper presented at the 2022 Aluminum Anodizers Conference: short.pfonline.com/UVanodiz
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