Question: Aluminum is to be pickled in hydrochloric acid. The purpose is a fine, technical structure with a tolerance of a few micrometers, which is required in a subsequent (non-galvanic) coating process. HCl pickling is rather atypical for aluminum and therefore we have not found anything about it in the literature so far. We are particularly interested in which pickling inhibitors / interfering substances are known. For example, the min. and max. concentrations of Al are known for the usual NaOH pickling processes. Is there anything like this for HCl pickles? How sensitive could the Al surface pickled with HCl be? If the parts are rinsed and dried after pickling and then stored without further coating, will there be any interaction with the air? Does an oxide layer form?
Unfortunately, we have no practical experience of this and cannot find much in the literature. Nevertheless, we will try to give you some thoughts and try to look at the subject from different angles. As the Al concentration increases, this will have an inhibiting effect and thus change the optical effect. We were unable to find any precise information on this, which is why a series of experiments in a beaker should help. It is also conceivable that copper has a disruptive effect even in small quantities, and there are even etching processes in which copper sulphate is required, at least for pure aluminum (see above). Pickling aluminum in hydrochloric acid is rarely used because it produces uneven structures. These are likely to vary greatly depending on the alloy and the load of the pickling. We therefore recommend trying out other processes that may lead to the same technical goal but offer greater process reliability.
Processes that were originally or primarily used for decorative purposes may help here. There are various options for decorative etching. A chemical process for matt etching mechanically polished aluminum or an alloy consists of immersion in a 5% ammonium hydrogen fluoride solution at 50 °C. For pure aluminum, 0.02 % copper sulfate is added here. The polished parts are cleaned in organic solvent and an alkaline degreasing solution, immersed in nitric acid and then placed in the etching solution. After the initial development of hydrogen, an aluminum fluoride film forms on the surface and the reaction comes to a standstill. After rinsing, the fluoride film is removed by immersion in 50% nitric acid and a 5 μm thick anodization is performed.
There is also the following method:
105 g/L ammonium fluoride
56 g/L hydrofluoric acid
15 g/L nitric acid
0.03 g/L lead nitrate
This produces a particularly attractive silky luster. This mixture prevents the crystal planes from being exposed and increases the etching effect. However, it consists exclusively of components that are undesirable today for environmental reasons. Uniform matting is achieved with an electrolytic process in which the surface is treated with alternating current in nitric acid or preferably hydrochloric acid. Due to adhering aluminum and aluminum chloride, the electrolytically salt-acid matted surface appears clearly and opaquely grey. The gray coating is incorporated into the layer during subsequent anodic oxidation, whereby the gray appearance is permanently retained. However, it can also be removed by pickling or chemical polishing before anodizing. The degreased and lightly pickled parts are immersed in a solution of 3 to 20 g/L (optimally 5 g/L) hydrochloric acid or 5 to 30 g/L (optimally 8 g/L) nitric acid and treated at room temperature for 4 to 15 minutes with alternating current (50 Hz) at 1.5 to 2.5 A/dm2. The initial voltage of 15 V rises to approx. 30 V by the end of the treatment. The process is also used for conditioning lithographic sheets and offset foils, as a preliminary stage for producing the so-called flitter finish or for producing anodically oxidized aluminium surfaces that can be permanently written on with ballpoint pens (e.g. for pendants). The flitter finish effect is characterized by a shiny and grainy surface, which gives it a glittering appearance and a very lively look. This is achieved by first pickling in caustic soda, then matting in an alternating current hydrochloric acid process, followed by polishing in a chemical or electrolytic polishing bath and anodizing using the GS process. Different variations can be achieved by changing the intensity and duration of the matting process.
The oxide layer
The oxide layer will form during drying at the latest, the first "layer" already in the sink. This is the reason why zincate pickles or stanate pickles are used for electroplating. This layer will grow over the duration of storage, whereby the speed depends primarily on temperature, surface condition (especially roughness) and humidity. This natural oxide layer is very dense but only a few nanometers thick. The layer structure and thickness vary depending on the moisture content of the air and the composition of the contact water. In air, the layer grows at a rate of about 1.5 to 2 nm per day up to about 10 nm. It continues to grow under the influence of the weather, but is then no longer transparent, but matt white. It is no longer the transparent boehmite, but hydrargillite or bayerite. On magnesium-containing alloys, the layer grows significantly faster at higher temperatures than on non-magnesium-containing alloys. The magnesium ions then diffuse outwards and form a MgO-rich layer on the surface. In humid air, the layer grows in a different form than in dry air. It then shows a similar shape to the technically produced layers. It consists of two different structures. The lower barrier layer is amorphous and consists of aluminum oxide. The upper top layer is porous and consists of aluminum oxide and hydroxide. Due to its porosity, it can absorb dirt particles, giving it a dirty gray appearance (weathering). Its total thickness is given as 4-10 nm. This is in addition to the topic "Sensitivity of the surface". The double layer also forms in water. Since the resistance of the aluminum really depends on the resistance of the passive layer, the physical and chemical properties of the various oxides, hydroxides and oxide hydroxides of aluminum must be taken into account.
A few words about the alloys: Historically, alloys were developed more in terms of their strength than their oxidizability. As aluminum is a soft metal, its application limits have been greatly extended as a result. Hardness is achieved by adding alloying elements (solid solution strengthening) and subsequent work hardening or heat treatment (hardening treatment). Alloys are therefore divided into age-hardenable and non-age-hardenable (naturally hard; AlMgMn, AlMn, AlMg). Precipitation hardening occurs when the alloying elements, and in particular their intermetallic compounds, accumulate in small areas. This leads to a lack of insulating strength of the oxide layer on the surface of the workpiece. This means that the oxide layer can also form differently with different alloys.
Depending on the alloy components, this can or will also affect the pickling/etching behavior. In addition, there are fluctuations in production (i.e. before you receive the parts) and, if applicable, impurities adhering to the surface that have diffused in or out. To ensure that you can maintain a consistently high quality - which is associated with very tight tolerances - the microstructure before HCl pickling must be as consistent as possible. Experience has shown that this only works if the entire process chain is coordinated and all companies/persons involved are aware of its importance. Otherwise it's "shit in, shit out". With tolerances as low as yours, it will probably also depend on the balance between HCl concentration and treatment duration. In our experience, we tend to reduce the concentration of the solutions and increase the treatment time - provided the same result can be achieved. In the opposite case (higher concentration, shorter time), the problem is that the more concentrated the pickling solution is, the more important the transfer times (into the sink) and room temperature become. So "haste makes waste" is the order of the day here.
Literature
[1] Recipes for metal dyeing, O. P. Krämer / T. W. Jelinek, Eugen G. Leuze Verlag, ISBN 978-3-87480-232-1
