![]() ![]() Deoxidizer Applicationsĭeoxidizers can be used in a variety of ways to help prepare aluminum surfaces for subsequent processing and to help salvage parts that have failed final inspection. These deoxidizers are very effective on casting alloys because of the fluoride’s ability to dissolve silica, a primarily chemically inert element that is used to make glass and in aluminum castings.Īll of these acid types are dangerous and proper operator safety precautions should be taken. Hydrofluoric-acid-based deoxidizers are aggressive and can be used on castings, or shot-peened or blasted parts, providing a light etch and a matte finish. 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. 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. 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 will passivate bare aluminum and tend to confer a passivating action on any other solution to which they are added. Tanks can be made of 316 stainless steel or polypropylene, or have some sort of polyvinylidene fluoride (PVFD) liner.Ĭhromic-acid-based non-etching deoxidizers are the gentlest of all deoxidizers. Nitric-acid-based deoxidizers are typically considered more environmentally friendly, too, since there is no chromium involved. Immersion time is 1-5 minutes, and triple rinsing is the most effective way of removing the solution. This is an effective deoxidizer/desmutting solution that can be used at room temperature without the need for fume exhaust. 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. They will produce satin-type finishes without removing the finished-metal shine. Nitric-acid deoxidizers are usually light-duty solutions and are principally employed as desmutting agents. 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 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. The most commonly used acid solutions for deoxidizing are nitric-, sulfuric- or chromic-acid based. This is why deoxidizing should be done right before anodizing or plating. ![]() 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. 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. Since ferric is in the +3 state, it can be the oxidizing agent and reduce the aluminum oxide on the surface. The key constituents in the chemical solution are ferric sulfate and nitric acid. 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. Thus, we need to remove it with a solution called a deoxidizer, which intentionally exceeds this pH range. This layer of oxide is what we seek in anodizing, but it is not of a sufficient thickness to serve our applications. 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. Details Peter Totaro, AeroTech Processing Solutions Cleaningĭesmutting 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.Ĭhemical processes common to preparing the metal for such surface finishing as anodizing, plating or painting put aluminum at risk for corrosion. ![]()
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