The relationship between sulfuric acid concentration and anodic oxide film thickness is not simply linear; rather, it involves the interplay of two competing mechanisms: the rate of oxide film formation and the rate of chemical dissolution. Within the standard concentration range (10%–30%) for sulfuric acid anodizing, film thickness increases with concentration, though the rate of growth gradually slows. At excessively high concentrations, chemical dissolution becomes the dominant factor, potentially leading to a decrease in film thickness or a deterioration in film quality. For the 6063 and 6061 aluminum alloys commonly used in keyboard casings, the sulfuric acid concentration is typically maintained between 180 and 200 g/L in mass production to strike a balance between film thickness and quality.
I. The Dual Mechanism of Film Formation
The growth of an anodic oxide film is essentially the result of a dynamic balance between two opposing processes.
Driven by the electric field, aluminum ions (Al³⁺) from the aluminum substrate migrate toward the electrolyte and combine with oxygen ions to form aluminum oxide (Al₂O₃); this constitutes the film formation process. Simultaneously, the sulfuric acid electrolyte chemically dissolves the oxide film that has already formed; this constitutes the film consumption process. The final thickness of the anodic oxide film is determined by the net difference between these two rates.
Concentration affects these two processes in opposite ways. Increasing the concentration raises the electrolyte’s conductivity, leading to higher current density at a given voltage and accelerating the rate of oxide film formation. However, higher concentration also enhances the sulfuric acid’s ability to chemically dissolve the oxide film. Consequently, the effect of concentration on film thickness is not monotonically increasing; instead, there exists an “optimal range.”
II. Experimental Data: How Concentration Alters Film Thickness
Early systematic experiments on the sulfuric acid anodizing process have provided quantitative evidence supporting this mechanism. A 1952 study examined the combined effects of concentration (10%–30%) and duration (20–60 minutes) on film thickness during DC electrolysis under fixed conditions (15 V, 16–17°C), yielding the following empirical formula:
1/T = 1.52C – 1.326 + 0.0053 + (5.6 – 0.13C)/t
Here, T represents film thickness (μm), C denotes concentration (%), and t stands for time (minutes). The formula reveals a complex non-linear relationship between film thickness and concentration, rather than a simple direct proportionality.
A subsequent study in 1953 further investigated the process under AC electrolysis conditions (15 V, 15°C), presenting a different empirical formula: T = (0.037C + 0.056)t + 0.05C + 1.1. The structure of this formula is more intuitive, showing that film thickness is derived from two components: a “time term” and a “concentration term.” For every 1% increase in concentration, the rate of film growth per unit of time increases by approximately 0.037 μm; simultaneously, the concentration itself contributes a time-independent baseline thickness (0.05C + 1.1).
Both studies reached a common conclusion: within the 10%–30% concentration range, increasing the concentration promotes greater film thickness, though the magnitude of this gain is modulated by time and voltage.
III. Limits and Risks of High Concentrations
There are clear limits to the positive contribution of concentration to film thickness. A 2026 study investigated the effects of medium-to-high concentration sulfuric acid (up to 12 M, or over 1000 g/L) on anodic oxide films. It found that the significant increase in film thickness observed in 12 M sulfuric acid was primarily attributable to the substantial incorporation of sulfate species into the film—causing volumetric expansion—rather than to the growth of aluminum oxide itself. This implies that an increase in film thickness at high concentrations may be accompanied by changes in the film’s composition and structure.
More importantly, excessively high concentrations can lead to a deterioration in film quality. A 2017 study on 6061 aluminum alloy clearly indicated that at high sulfuric acid concentrations of 3–5 M (approximately 300–500 g/L), vigorous electrolytic reactions cause surface cracking, actually resulting in reduced corrosion resistance. Excessive concentrations cause the dissolution rate to exceed the formation rate, rendering the film loose and porous, or even leading to “burning.”
Therefore, there is no simple positive correlation between concentration and film thickness. In engineering applications, the goal is to achieve a balance where the formation rate slightly exceeds the dissolution rate, rather than simply maximizing the concentration.
IV. Engineering Practice for Keyboard Casings
For the 6063 and 6061 alloys commonly used in CNC-machined aluminum keyboard casings, the typical sulfuric acid concentration range for anodizing is **180–200 g/L** (approximately 18%–20%). This range is derived from extensive industrial experience: the concentration is high enough to ensure a reasonable film growth rate and good dyeability, yet not so high as to compromise film quality through excessive dissolution.
The concentration strategy for hard anodizing (Type III) differs. Traditional hard anodizing employs higher sulfuric acid concentrations (typically 200–250 g/L) combined with low temperatures (-3°C to 0°C) and high current densities to suppress dissolution and promote the growth of a dense film. However, low-concentration hard anodizing methods are also being developed; some studies have utilized very low sulfuric acid concentrations (around 2%) combined with pulsed power supplies at temperatures between -2°C and 0°C to achieve excellent surface uniformity.
It is important to emphasize that concentration is only one of the variables affecting film thickness. Current density has a more direct and linear impact on thickness—research on hard anodizing 6061 aluminum alloy shows that “current density linearly affects film thickness,” whereas concentrations in the 120–180 g/L range have no significant effect on the salt spray resistance of the film after sealing. This implies that in actual production, controlling film thickness via current density is more reliable than doing so via concentration; the primary role of concentration is to maintain the electrolyte’s conductivity and process stability.
V. Concentration Control in Mass Production
For the anodizing line used for keyboard housings, the core objective of concentration management is not to adjust concentration in order to alter film thickness, but rather to maintain concentration stability. As the anodizing process proceeds, sulfuric acid is consumed and aluminum ions continuously dissolve into the bath, causing the concentration to drift gradually. If the concentration falls out of control, the consistency of the film thickness cannot be guaranteed.
A technical document from 1985 explicitly states that during the aluminum anodizing process, the sulfuric acid concentration in the electrolyte affects both the film thickness and the properties of the coating. It notes that—provided current density, temperature, and anodizing time meet specified parameters—implementing automatic control of the sulfuric acid concentration in the anodizing bath ensures consistent oxide film thickness. This demonstrates that in a mass production context, the goal of concentration control is to eliminate fluctuations rather than to actively adjust film thickness.
In the mass production of keyboard housings, anodizing facilities typically maintain the concentration within the target range by periodically analyzing the bath solution and replenishing sulfuric acid and deionized water. Actual control of film thickness relies on the precise management of current density and anodizing time, as well as spot checks of film thickness on each batch of products.
