Appunti Surface Engeneering

ZINC PHOSPHATE ON ALUMINUM

A number of methods of phosphate treating aluminum prior to painting have been utilized, including iron, zinc, chromate and manganese-modified zinc phosphating processes. Due to environmental concerns (hexavalent chromium is a toxic ion), and for lower overall operational costs, some chromic-phosphates have been replaced with iron or manganese modified zinc phosphates. These pretreatments are intended for moderate service conditions.

These phosphates can produce acceptable quality coatings on parts manufactured with both steel and aluminum components. A major drawback to some formulations is low tolerance for dissolved aluminum, which reduces the life of the process.

TROUBLESHOOTING ZINC PHOSPHATING 08/04/2020

ANODIZING ALUMINIUM AND ITS ALLOYS

Anodizing defines an electrochemical treatment whereby stable oxide films or coatings are formed on the surface of metals: a sufficiently high potential (voltage) is applied to establish the desired polarization to deposit oxygen.

(oxygen overvoltage) at the surface where it reacts with the substrate to form an adherent oxide (anodic) coating.

Why to anodize materials?

• Increase the CORROSION RESISTANCE
• Increase the WEAR RESISTANCE since oxides are harder than the metal itself
• METALS that CAN BE ANODIZED:

aluminium and its alloys are the most common, we will focus on them.

titanium has the characteristic that the colour is given by the thickness of the oxide layer

tantalum

magnesium is nowadays in very interest nowadays

These metals can be anodized since their oxides are enough chemically stable to resist in almost every environment (in case of iron or nickel the oxides are not stable enough).

Anodic coatings can be formed on aluminum employing a wide variety of electrolytes utilizing either AC or DC current or a combination of both.

Anodic coatings are classified according to the solvent action of the particular electrolyte on the anodic oxide produced in the reaction:

Anodic coatings produced in SULFURIC

ACID (most common) electrolytes are of the porous type because the electrolyte employed has a solvent action on the anodic oxide coating; it can be grown in various thicknesses (from microns to 0.1 mm).

CHROMIC ACID has a similar effect to the sulfuric one, but it is no longer used due to its high toxicity.

PHOSPHORIC ACID (or combined with sulfuric acid) has even greater solvent action on the coating, resulting in films with high porosity; these films are used as a base for plating or adhesive bonding on to aluminum surfaces.

OXALIC ACID

SULFO-ORGANIC ACIDS

BORIC ACID produces films relatively thin, being approximately 0.02 mils in thickness, and essentially nonporous. Such films are called "barrier" anodic films and are used commercially for their unique electrical characteristics for such applications as electrical capacitors. Also they serve as a protective film for vacuum deposited aluminum on precision mirrors for optical equipment.

Difference between ANODIZING

In electroplating the coating is provided by the adhesion of a deposit, with an increase of the size of the component, with some problems in thickness distribution (edge/corner effects: there the current lines accumulate and the thickness is more significant).

In the case of anodization, we have a dissolution reaction at the anode, so the size of the actual metallic part is reduced; the dissolved material then reacts with the oxygen generated at the electrode to form the oxide layer.

However, since the oxide coating has a greater volume than the aluminum substrate, there is an overall increase in dimensions (from 1/3 to 1/2 of the coating thickness) -> I have to consider it industrially if I have strict size requirements and for stripping and re-anodizing processes (this is a drawback with respect to plating).

In case of anodization the distribution of thickness is much more uniform since the oxide is an insulator: when it grows, the ohmic resistance increases locally,

favouring the growth in places where the oxide is less thick (SELF-REGULATING GROWTH -> we can grow conformal coatings more easily).

Anodizing ALUMINUM vs ALUMINUM-ALLOYS

1. Usually, aluminum alloys are formed by an aluminum matrix with a dispersion of stoichiometric compounds of aluminum and the alloying element; these stoichiometric compounds are also present at the surface, and their reactivity is practically null when compared to the one of aluminum. This leads to the formation of macro defects, so in the case of aluminum alloys the PRE-TREATMENT STAGE is fundamental: not only de-greasing, but also the etching step (typically acidic) -> this removes the oxide layer present at the surface the precipitates as well, allowing us to form a uniform and compact protective layer.
2. I have to consider that different alloys in the same conditions will have different conductivity, therefore the film will have different growing rates.

Chemical composition of the anodic film

STRUCTURE of the oxide

THE ROOM-TEMPERATURE (see below).

The VOLTAGE SHOULD BE APPLIED IN A RAMP MODE since at the beginning of the process the conductivity of the substrate is so high that the formed film would be uncontrolled, forming defects and non-uniform film (BURNING).

The growth of the film should be linear, but the actual growth presents a plateau since some chemical dissolution of the oxide is chemically dissolved by the action of the electrolyte (very acidic).

The limit value represents the equilibrium between the grows of the film and the chemical dissolution (which rate increases as the film grows thicker since the exposed area grows).

The aggressiveness of the electrolyte can be reduced by reducing the temperature, increasing the amount of oxide we can grow even at plateau, reaching the theoretical grow at very low temperature (note that anodic film weight is bonded to the film thickness by the area).

Industrially maintaining the temperature between 0°C is complicated, so we usually work in the order of

3-4°C.PULSE ANODIZING is possible to tailor the characteristics of the coating.

SOFT & HARD anodizing

Those adjectives refer to the hardness of the coating we are producing: the hardness is related to the kind and abundance of porosity and the presence of contaminants such as water or aluminum sulphate usually.

I can control the hardness by modifying the working parameters: The breakthrough voltage and the temperature resistance will also be increased with respect to the softer coatings.

Anodizing CONFIGURATION

Anodization can be produced by STATIC CONFIGURATION (in tank or basket [sort of barrel but not rotating]) or by CONTINUOUS CONFIGURATION (coil anodization). The racking should be done properly, avoiding the formation of bubbles between the parts.

The coil anodization is fundamental in many industries: food industry (packaging), micro-electronics industry (electrolytic capacitors)...

Being an electrochemical process, we need an ANODE (part to be anodized) and a CATHODE: the latter can

be aluminum itself, titanium or stainless steel, since it will be protected by the cathodic polarization during the process and therefore will not dissolve or take part in the reaction, but will only be the place where hydrogen molecules will be reduced.

Anodizing PROCEDURE

Anodization steps (REMEMBER: after any step we need RINSING):

1. CLEANING: removal of the organic contaminations (de-grease of the part)
2. ETCHING: acid etching is always performed after the alkaline one in order to remove some precipitates that form during the alkaline etching.
1. ALKALINE ETCHING: aimed to remove the native oxide layer
2. ACIDIC ETCHING: to remove the precipitates from the surface
3. There is also the possibility to apply a chemical etching (BRIGHT DIP = brillantatura), in order to reduce chemically the roughness. With aluminum is quite easy to achieve in this way a very flat and therefore bright surface, widely used for example in the back of lamps.
3. ANODIZATION: after this step we have a component covered

are preferred since they also provide a sort of anti-bacterial barrier; some processes are nowadays under study that slowly release anti-bacterial and anti-viruses substances, such as copper.

There is also the possibility to apply the colour during the anodization process, but it is more complicated and less common.

POST-SEALING TREATMENT: for all of them timing is in the order of minutes.

NICKEL ACETATE SEALING: most common industrially due to its low cost. Immersion process (no current) in a water-based solution with nickel acetate salt, then rinse the component and dry it. In this way nickel hydroxide and acetates species will precipitate inside the pores; the nickel hydroxide species are large in size and typically insoluble, so we