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Deoxidizing Aluminum as a Pretreatment
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Deoxidizing Aluminum as a Pretreatment
Desmutting is the act of removing excess alloyed metals from the surface of aluminum after etching, and it can be done using any mineral inorganic acid, such as hydrochloric, sulfuric and nitric acids. Deoxidizing is different from desmutting in that it removes aluminum oxide off the aluminum substrate via redox (or oxidation-reduction) reactions, something normal mineral acids cannot achieve. You can always desmut aluminum, but you may not always be able to deoxidize aluminum prior to processing.Get more news about Acrylic Deoxidizer,you can vist our website!
Chemical processes common to preparing the metal for such surface finishing as anodizing, plating or painting put aluminum at risk for corrosion. Interestingly, aluminum inherently protects itself from this corrosion in stable conditions (pH of around 4.5-8.5) by readily oxidizing—creating a very thin layer of aluminum oxide when it comes in contact with the surrounding oxygen. This layer of oxide is what we seek in anodizing, but it is not of a sufficient thickness to serve our applications. Thus, we need to remove it with a solution called a deoxidizer, which intentionally exceeds this pH range.
Of course, deoxidizing of aluminum can only be successful if there is aluminum oxide present on the part or there is a chemical species willing to donate or accept electrons in the solution. The key constituents in the chemical solution are ferric sulfate and nitric acid. Since ferric is in the +3 state, it can be the oxidizing agent and reduce the aluminum oxide on the surface. The nitric acid is very important because of its inherent oxidizing abilities, which help speed up the reaction and decrease the amount of time the part spends in the deoxidizer. Once the thin layer of aluminum oxide is gone, you are left with bare aluminum that has been activated, meaning it is looking to donate electrons to maintain equilibrium. This is why deoxidizing should be done right before anodizing or plating.
Types of Deoxidizers
The most commonly used acid solutions for deoxidizing are nitric-, sulfuric- or chromic-acid based. Nitric and sulfuric solutions are usually interchangeable or paired, so I will only discuss nitric-based deoxidizers, which are more common, and chromic-acid based ones.
Nitric acid is the more common choice for non-etching deoxidizing because of its ability to slowly attack aluminum and because of its autocatalytic abilities as an oxidizer. Nitric-acid deoxidizers are usually light-duty solutions and are principally employed as desmutting agents. They will produce satin-type finishes without removing the finished-metal shine. Nitric-acid deoxidizers also are useful for salvaging parts, as nitric acid is known to open the pore structure of anodic films, allowing for easier stripping of anodize.This is an effective deoxidizer/desmutting solution that can be used at room temperature without the need for fume exhaust. Immersion time is 1-5 minutes, and triple rinsing is the most effective way of removing the solution. Nitric-acid-based deoxidizers are typically considered more environmentally friendly, too, since there is no chromium involved. Tanks can be made of 316 stainless steel or polypropylene, or have some sort of polyvinylidene fluoride (PVFD) liner.
Chromic-acid-based non-etching deoxidizers are the gentlest of all deoxidizers. They will passivate bare aluminum and tend to confer a passivating action on any other solution to which they are added. These types of deoxidizers are popular for removal of heat-treat films and to prepare aluminum alloys for zinc immersion plating, chromic-acid anodizing, painting and other chemical treatments. They help eliminate surface irregularities caused by oxide inclusions or embedded buffing particles, and generally do not remove nearly as much metal as caustic soda while leaving the surface of the metal clean and semi-bright. Chromium-based deoxidizers work best in tanks made from 18-8 stabilized, stainless steel-clad material or lined with lead, PVFD or high-density polypropylene.