Factors Affecting the Alkali Corrosion Rate of Aluminum Profiles and How to Control It

To speed up the alkali etching of aluminum profiles, you can add sodium nitrate or sodium nitrite, and then additives like sodium gluconate, to a sodium hydroxide-based alkali etching solution. Besides that, factors like the concentration of the solution, temperature, and etching time also affect the speed of industrial aluminum profile alkali etching, as well as how to control these factors.

Tank Solution Concentration

Dissolving sodium hydroxide in water creates an alkaline solution. This kind of solution is usually used as a chemical cleaning tank solution for aluminum profiles, also known as alkaline etching solution. In the alkaline etching solution, the sodium hydroxide exists in free ion form. As the concentration of sodium hydroxide increases to 250 g/L, the etching speed of industrial aluminum profiles also rises. This shows that the higher the sodium hydroxide concentration in the tank solution, the faster the etching speed. However, many companies nowadays control the sodium hydroxide content to be within 50–100 g/L to save costs. To control the sodium hydroxide content, you can add long-life additives like gluconates to the tank solution, keeping the free sodium hydroxide content around 70 g/L.

Aluminum Dissolution Amount

When aluminum profiles are placed in sodium hydroxide alkaline solution, aluminum reacts with sodium hydroxide to form aluminum hydroxide. The formation of this product slows down the etching speed of aluminum. Experiments have shown that when the aluminum dissolution amount is 26 g/L and the sodium hydroxide concentration is 50 g/L, the higher the temperature, the slower the aluminum etching speed. This indicates that if the tank solution concentration remains the same, the higher the aluminum dissolution amount, the slower the etching speed, meaning the aluminum dissolution amount is inversely proportional to the etching speed. It’s important to calculate the appropriate aluminum dissolution amount, carefully observe changes in the tank solution, and adjust the aluminum dissolution amount within the specified range as needed.

Solution Temperature

Through experiments on 6063 aluminum profiles, we concluded that as the solution temperature rises, the corrosion rate of aluminum in alkali also increases rapidly. For every 10°C increase in temperature, the rate of aluminum corrosion doubles. So the higher the temperature, the faster the aluminum profile corrodes—in other words, the solution temperature is directly proportional to the corrosion rate. This happens because the corrosion of aluminum in an alkaline solution is an exothermic reaction that releases up to 20 kW/m² of heat. Usually, a cooling system with enough capacity is needed to keep the solution temperature within 50-60°C. This system can also function as a heater in the winter.

Corrosion Time

In the alkaline corrosion reaction, the longer industrial aluminum profiles are exposed, the more weight they lose due to aluminum dissolving. However, corrosion time doesn’t directly relate to the corrosion rate. Shortening the corrosion time only reduces the weight loss of aluminum and decreases the consumption of alkali chemicals, but it doesn’t proportionally affect the corrosion rate. To achieve a smooth aluminum surface, you can mechanically brush or sandblast it first, or improve the mold quality and extrusion process to get a bright, even finish.