Metall- und Oberflächenchemie Sperzel GmbH

Aluminium Anodizing Technology

Chemicals, Processes, Equipment

Product information

Etch Additive Al 7000

Long-life etch additive for decorative surface treatment according to DIN 17611/E6

Al 7000 is free from nitrite, nitrate, chlorate and chromate. The etching additive is used in conjunction with sodium hydroxide to produce matt surfaces, whereby press marks and minor scratches are largely leveled or concealed
Al 7000 contains a higher proportion of heavy metal precipitants and is therefore particularly suitable for alloys made from recycled material with a higher heavy metal content.
Al 7000 keeps the concentration of heavy metals in the solution at a very low level and thus largely prevents the typical coarse grain effect.
In its application, Al 7000 is characterized by an outstanding runout behavior and clearly reduced foam formation. These properties allow significantly higher aluminum contents in the etch liquid in the range of 180-190 g/l aluminum. Floating of the flight bars, well known in case of high aluminum contents, is suppressed by Al 7000.

1. Delivery form, standard packaging, storage

Delivery form

  • liquid
  • Al 7000 is dosed as delivered

Packaging

  • IBC à 1200 kg
  • PE-jerrican à 36 kg

Storage

  • frost-free
  • avoid direct sunlight

2. Procedure

Line sequence

  • Degreasing
  • Rinse (may be not required - depending on the application)
  • E6-etch (Al 7000)
  • Warm rinse
  • Further rinsing steps
  • .....

3. Plant equipment

Bath tank

  • Tanks should be made of mild or stainless steel. Since the working temperature range is up to 70°C isolation is advantageous.

Heating

  • The etch reaction is strongly exothermic. During normal operation and when the E6-etch bath is occupied permanently heating is hardly necessary. However, after downtimes (e.g. weekends), if the bath is used only irregularly or to keep the bath within a safe temperature range during maintenance, heating is essential.

Cooling

  • Due to the strong heat development cooling is essential to keep the etch liquid within the desired temperature range also for large loads. Heating and cooling should work harmoniously with each other and not run against each other at the same time.

Bath circulation

  • In general, we recommend the installation of an air pipe. Under normal working conditions the gas development during the etch process ensures good mixing of the liquid. However, if larger quantities of water or chemicals are added, air mixing is helpful. Likewise, during longer shutdown periods, the etch should be recirculated from time to time to reduce the settling of sludge.
  • Permanent air injection is normally not required.

Bath exhaustion

  • Bath exhaustion is required to extract the alkaline etch aerosols.

Crane system

  • No special requirements are imposed on the crane system. However, a good slope is advantageous to allow the viscous etch solution to drain off quickly after lifting, as otherwise narrow hollow profiles may clog.

Filtration

  • Filtration of the etch bath is not required.

4. Make-up and operation of the bath

Preparation

  • Normally, existing E6-etches can be overrun. The previously used etch additive is simply replaced by Al 7000. Likewise, the usual analysis methods are usually suitable, although we may suggest newer or simpler determination methods.
  • If you have any questions about analyses and dosing, please do not hesitate to contact our application engineers or laboratory.

Bath make-up

  • A new make-up is ideally done by a ready-to-use etch, which only has to be filled into the tank and brought to operating temperature. After analysis and (if applicable) correction of parameters the E6-etch can be integrated into the production process.
  • If a new preparation of the solution including aluminium enrichment is necessary, please consider the following quantities per m³ bath volume:
  • Aluminium min. 100 kg
  • Caustic soda 50% approx. 400 kg
  • Al 7000 30–40 kg
  • Depending on the plant conditions (heating, cooling, exhausting, laboratory availability etc.), at least 48 hours of continuous operation must be estimated for enriching the etch to 100 g/l aluminium. The schedule and details of the make-up should be discussed with our technical department.

Bath temperature

  • The extended working range is between 55 and 70°C
  • In order to achieve a high matting degree with optimum metal removal a temperature of 68 +/- 1°C is recommended. Due to limiting plant conditions, such as foam development at high bath levels, lack of overflow tank, small bath volume with large etched surface, weak cooling capacity etc., a different temperature range may be selected.
  • However, in order to minimize deviations from batch to batch in the etch finish, the se-lected working temperature should be kept as constant as possible. A maximum tolerance of +/- 1°C should be realized.

Treating time

  • This depends on the desired matting degree and given parameters and is usually between 8 and 15 minutes, depending on the requirements. Significantly longer immersion times do not improve the matt finish but lead to unnecessary metal removal only.
  • Like the temperature, also the immersion time has a significant influence on the etch result. For good reproducibility, the etch station should have priority regarding crane availability

Rinsing

  • The E6-etch is usually followed by a hot rinse to remove adhering etch residues more efficiently. Temperatures between 35 and 40°C show the best effect in practice. A certain degree of alkalinity is also advantageous to start a slight etch attack to blast off adhesions from the surface. The aluminium content in the hot rinse should be within the range of 30 - 40 g/l. Low aluminium-contents make the hot rinse more active and raise the removal rate; higher concentrations decelerate the system and deteriorate the rinsing effect.
  • For optimum performance of the hot rinse, the 3 parameters NaOH concentration, temperature and aluminium-content must be coordinated and adjusted to the plant conditions.
  • The other rinsing steps should be arranged in cascade to optimize water consumption. The last rinse before the desmut is suitable as a collar rinse to remove adhering aluminium-residues also from the bus bars.

Bath monitoring

  • The E6-etch should be monitored according to a defined analysis plan. Temperature and bath level must be checked regularly, which can also be done by automatic control.

Bath control

  • -NaOH 40–60 g/l (turbidity point method)
  • Rule of thumb: 1/3 of Al content - 5g/l = target value NaOH (+/- 3 g/l)
  • For example, 160 g/l Al corresponds to a target value of 50 - 56 g/l NaOH
  • -Al 7000 30–40 g/l
  • -Aluminium own value specified, but for E6 at least 120 g/l.
  • The aluminium-content is usually between 120 and 180 g/l. Higher contents allow better matting with lower metal removal. The maximum aluminium-content for safe and trouble-free production depends on the individual plant conditions and has to be determined in practice.
  • As the aluminium-content has a significant influence on the etch finish, the range should be specified and should not vary by more than +/- 5 g/l.
  • The NaOH-content varies the most, hence we recommend one analysis per shift. Aluminium and Al 7000 should be checked 1x per day.
  • We will be pleased to provide you with the analysis specifications on request.

Replenishing

  • Automatic dosing is possible since both the additive and caustic soda are available as liquids. To keep the concentrations as constant as possible, we recommend automatic dosing per bar or ideally per area (e.g. every 50, 100 or 200 m² depending on the size of the system). Experience has shown that the recommended replenishment quantity is 30-35 g/m².

Stone prevention

  • Too low concentrations, both of caustic soda and of Al 7000, can lead to undesirable scale formation. For sufficient complexation of aluminium, the concentrations of NaOH and Al 7000 must be kept within the target range. Even temporary underconcentrations can lead to deposits.

Maintainance work

  • (e.g. plant maintenance), the stability of the etch can be increased by various measures to prevent the so-called collapse:
  • - Raise NaOH-content by 10-20 g/l
  • - Increase additive-content by about 10 g/l
  • - Keep the temperature at 45-50°C
  • - Switch on the air circulation at least 1x per day and mix the bath well to reduce sludge formation.
  • Please be aware that when production is resumed, the bath reacts more strongly due to the increased caustic soda-content and changes in the etch finish may result. Consequently, sensitive material should not be etched within the first few hours until the usual NaOH-concentration has been achieved again.

Lifetime

  • If run constantly within the recommended concentrations and usual cleaning intervals, the bath can be operated without time limitation

Runout behavior

  • In case of special requirements, e.g. etching of profiles with narrow hollow sections, the runout behavior can be improved by adding the wetting agent Al-N Plus.
  • If the requirements on the runout behavior in the plant increase permanently, e.g. due to a changed product mix, the etch additive can generally be changed or adjusted. Before a changeover, however, our technical department will discuss the specific conditions in your plant with you since a changeover can also have disadvantageous aspects.

Bath poisons

  • The addition of zinc should be avoided, since uneven surfaces or grain effect may result. Special ingredients in the etch additive AL 7000 intercept the heavy metal components of common aluminium alloys and precipitate them harmlessly for the process. If alloys with higher zinc-contents are etched regularly, it is possible to adjust the etch additive accordingly.

Wastewater

  • Concentrates, bath solutions and rinsing water must be treated in accordance with the legal treatment regulations before being discharged into the sewage system.

Occupational safety measures

  • Whenever handling chemicals, the usual safety precautions must be applied
  • Personal protective equipment (PPE) must be worn.
  • For details in handling please refer to our Material Safety Data Sheet (MSDS)

5. Process specific safety instructions

Preventive fire and explosion protection in etching processes

Due to the development of hydrogen gas during etching, the requirements of preventive fire and explosion protection must be observed.
To determine the hazard situation in your anodizing plant and to prepare a legally required explosion protection document, we recommend contacting a specialist company for fire and explosion protection.