Aluminium anodising

Alkaline etching of aluminium

Long life alkaline etching bath

For decorative matting of the surface during anodizing of aluminium products in Europe, alkaline etching is mainly used. These special pickling baths are used in accordance with DIN 17611 for surface treatment “E6”. Such pickling baths are usually called so: “E6 etching baths” or “long life etching baths”.

These long life etching baths are very long-term, if properly maintained. Correct maintenance of these baths is especially important then, when they have a high aluminium content, what is needed to achieve a decorative matte surface. This prevents the bath from going into an unstable state. Insufficient maintenance of the bath can lead to precipitation of aluminium hydroxide, which means the complete loss of this bath.

The main features of the long life etching bath are:

  • High aluminium content
  • Specified sodium hydroxide content in relation to aluminium content
  • Special additives for keeping aluminium in solution
  • Additives for the deposition of impurity metals.

Impurity metal precipitation additives separate the impurity metals from the aluminum alloy and form with them insoluble or poorly soluble metal sulfides (usually very small, black particles). Depending on the content of impurity metals in the aluminium alloy, the alkaline solution of the E6 bath quickly turns black. Small, black pigments are not involved in the etching process [1].

Chemistry of alkaline etching of aluminium

Purpose of Alkaline Etching of Aluminium

The process of alkaline etching of aluminium products occurs when they are immersed in an aqueous solution of sodium hydroxide and some additives.

Factors, which affect this process are:

  • Etching rate, which depends on:
    – temperature
    – aluminum hydroxide concentration
  • Time (processing time).

The purpose of alkaline etching is:

  • Dissolve the aluminium surface to remove embedded impurities and imperfections.
  • The result is a smooth, homogeneous base for the subsequent anodizing process to obtain an anodic coating with a decorative finely dispersed matte surface.

Chemical reactions of alkaline etching of aluminium

It happens, when aluminium on the surface of the product interacts with sodium hydroxide and water to form sodium aluminate and release hydrogen, according to the following chemical reaction (reaction # 1) [2]:

2Al + 2NaOH + 2H2O = 2NaAlO2 + 3H2

This reaction occurs with the consumption of free sodium hydroxide.

When the concentration of sodium aluminate increases, the side reaction according to the following formula (reaction # 2) becomes more and more preferable:

NaAlO2 + 2H2O = Al(OH)3 +NaOH

In this reaction, sodium aluminate reacts with water, resulting in the formation of aluminium hydroxide, which accumulates as a suspension in the bath solution E6. As you can see from this formula, this reaction also results in the release of additional sodium hydroxide, which the, in turn, consumed in reaction # 1.

Aluminum hydroxide slowly settles from a suspension in the E6 bath solution to the bottom of the bath or to various surfaces inside the E6 bath, for example, on heating elements, very hard fossils. This occurs according to the following reaction of aluminium hydroxide dehydration (reaction # 3):

2Al(OH)3 = Al2O3 +3H2O

The resulting reaction for the aluminium alkaline etching process is a reaction between aluminium and water to form an aluminium hydroxide precipitate and release hydrogen.

2Al + 6H2O = 2Al(OH)3 + 3H2

Factors, which affect the rates of these reactions # 1, # 2 and # 3 are:

  • temperature
  • sodium hydroxide concentration
  • aluminium concentration in solution
  • additives to prevent precipitation of aluminium hydroxide from solution and to promote desired reactions

When the concentration of aluminium in solution increases, free aluminium hydroxide is consumed according to reaction No. 1. It can quickly reduce the sodium hydroxide concentration until, until reaction # 2 releases it back into solution.

Solubility of aluminium in an alkaline solution

Aluminium dissolves well in solutions of caustic soda [3]:

  • 7 moles per liter, which is equivalent to:
  • 7 x 27 = 189 g / l for Al3+ or
  • 7 x 82 = 574 g / l for NaAlO2

The numbers 27 и 82 Is the atomic weight of aluminium and the molecular weight of sodium aluminate, respectively.

Such a high solubility of aluminium in a caustic soda solution is achieved when the following conditions are met simultaneously:

  • the specified concentration of caustic soda (NaOH) is maintained,
  • the set temperature of the solution is maintained,
  • the specified concentration of special additives is maintained, which prevent the precipitation of aluminium hydroxide.

Maintaining the stability of the E6 bath

A high aluminium content is required for particularly matt surfaces, which can reach 200 g / l. With such a high aluminium content, special attention is required to bath stability [1].

Stable black E6 bath

Just one look is enough, to determine if bath E6 is still stable. If it has a deep black color (picture 1), then you can be sure, that the bath is sufficiently stable. Of course, this visual assessment must always be confirmed by regular chemical analysis of the composition of the bath.

Figure 1 – The black E6 bath means the stable condition [1]

Gray E6 bath: What’s going on?

If the E6 bathtub turns gray instead of rich black, then you need to react immediately. In this case, the process of destabilization has already begun. During this process, white, poorly soluble aluminium hydroxide particles. If the proportion in the bath continues to increase, then the bathtub becomes more and more gray, that is, the black color changes to dark gray, then medium gray, then light gray, until it turns completely white. In a light gray and white state, the bath is considered lost and must be completely restored., which requires significant amounts of finance, work and time. Therefore, it is necessary in every possible way to avoid the occurrence of such a state.

Figure 2 – The dark gray E6 bath means the beginning of destabilization [1]

How to save the unstable gray E6 bath?

If the alkaline bath E6 turns slightly gray, necessary:

  • Complete bath chemistry analysis immediately.
  • A decrease in the aluminium content will indicate the instability of the bath, since part of the dissolved aluminium has already separated from the solution in the form of poorly soluble aluminum hydroxide.
  • At the same time, the sodium hydroxide content will increase, so it is released during the destabilization of the solution.

Figure 3 – Chemical reaction of the destabilization process [1]

It would be a mistake not to add sodium hydroxide to the bath, although its content is most likely still within the specified interval. Low alkalinity contributes to further destabilization of this bath [1]. So here’s what to do:

  • A large amount of a special additive must be added to the gray bath (for example, Alfasatin) for extra stabilization, for example, twice as much, than usual.
  • It is recommended to increase the alkalinity of the bath by adding sodium hydroxide, to further stabilize it.

These stabilization measures ensure that, to:

  • no further precipitation of aluminium hydroxide occurred and
  • the aluminium hydroxide remaining in the bath solution was gradually removed from the bath with treated aluminium surfaces.

As a result, the bath should gradually return to a stable state (black). During this time, monitoring the bath chemistry is especially important. If the bath solution turns black again, then the stabilization process can be considered successful.

What to do with a light gray or milky white E6 bath?

If the E6 bath has reached this state (Figure 4), then there is no longer any way to save her. You can only wait a while, until the white sludge settles to the bottom of the bath. The settled liquid phase can then be pumped into an empty bath or container and then, depending on her condition, use when preparing a new bath.

However, the released white alumina sludge must be removed from the bath as soon as possible. If you delay with this, it will become stone and it will already be very difficult to clean the bathtub from it (Figure 5).

Figure 4 – Fully destabilized E6 bath [1]

Figure 5 – The petrified sludge of aluminum hydroxide from the lost E6 bath E6 [1]

Sourses:

  1. https://alufinish.de/en/destabilisation-of-alkaline-e6-long-term-pickling-processes/
  2. SPECULAR REFLECTANCE OF ANODIZED 6061-T6 ALUMINUM ALLOY / JON STRAUSS, Oregon State University – 2013
  3. Alkaline Etching of Aluminum & Its Alloys – A New Caustic Soda Recovery System –Italecno S.r.l. & Chemetall 2002.