anodizing basics

7/29/2019 Anodizing Basics

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ANODIZING BASICS

Anodizing of aluminum can achieve a very durable surface that isunaffected by weather for many years and many chemicals if done properly.The anodized surface can be dyed into many different colors using organicdyes. These colors can also be mixed to provide a full spectrum of othercolors. The dyed surfaces can be subsequently sealed in a hot sealsolution to protect away from corrosion in the surrounding environment. .

MECHANICAL & CHEMICAL SURFACE TREATMENTS

Aluminum can not be anodized and dyed without preparation. Themechanical and chemical preparatory treatments determine the finalappearance of the metal surface since its optical properties are notmodified by the transparent anodic coating or by dyeing. The mechanicaltreatment may include buffing, grinding, sandblasting and etc. Thechemical treatment is to clean the metal surface free from oils, occludedimpurities, soap residues and the natural oxide film.

ANODIZING

A porous anodic coating is formed during the anodizing process. Theporous film can adsorb dyes to produce a full spectrum of colors. Thenatural oxide film is very thin and has to be replaced by a thicker artificialformed film. The most popular method of anodizing is to use a directcurrent (D.C.) sulfuric acid process. The following table is the one mostused in the industry.

Sulfuric acid (free) : 165 g/l 225 g/

Aluminum content : 5 15g/l

Current density : 1.2 1.8 A/ dm2

Voltage : 12 V 20 V

Current : D.C.

Temperature : 18 22C

Time : 25 40 minutes

Coating thickness : 8 12 um

The following table gives a guide line to the effect of the D.C. sulfuric acidprocess parameters on the dyeing properties of anodic coating :-

Parameters Deviation Dyeing Intensity MechanicalStrength

H2SO4 Conc. Higher Deeper Lower


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Lower Paler Higher

Aluminum ions Higher Modified CoatingStructure

Modified CoatingStructure

Lower Paler Higher

Current Density Higher Paler Higher

Lower Deeper Lower

Temperature Higher Deeper Lower

Lower Paler Higher

Time Longer Deeper Higher

Shorter Paler Lower

CoatingThickness

Thicker Deeper Higher

Thinner Paler Lower

Remarks

Sulfuric acid concentrationAn increase in the sulfuric acid concentration intensifies its dissolvingaction on the metal, producing pores of more markedly comical shape andof greater average diameter. As the increased porosity provides a greater

inner surface area, the adsorptive capacity is enhanced allowing dyeing ofgreater intensity to be produced. During the service life of the anodizingsolution more and more free sulfuric acid is expended in dissolving, and asthe concentration declines so does the absorptiveness of the coating. Inorder to ensure that successive dyeings are of equal intensity, it is mostimportant to maintain the free sulfuric acid concentration within a narrowlimit of 160 225 g/l.

Aluminum contentThe dye adsorptive capacity decreases sharply below 5g/l AL, but thenremains nearly constant at higher concentration. At over 15 g/l the solution

does not work efficiently and there are likely to be irregularities in theanodic coating. So in order to achieve color uniformity, it is advisable tokeep the content between 5 g/l to 15 g/l.

Current densityAt high current density and constant coating thickness the dye adsorptivecapacity is lower since the metal is exposed for a shorter time to the


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dissolving action of the sulfuric acid. In joint anodizing of alloys whichdiffer in composition, fluctuations in current density may occur betweenone part and another which are reflected in color differences when theparts are dyed. For this reason, only one alloy should be anodized in agiven batch : this is particularly important if the parts are to be dyed.

TemperatureA rise in temperature intensifies the dissolving action of the sulfuric acidresulting in softer, more porous coatings with higher capacity of dyeadsorption. As the temperature has a very marked effect, close control is

imperative. The tolerance limits are +/- 2C.

In this connection it should be noted that if the heat convection isinadequate, local temperature variations occur which later will result inuneven dyeings. To safe guard against the occurrence of local overheatingthere should be provision for thorough agitation and cooling of theanodizing solution.

Anodizing time and coating thicknessThe thickness of the anodic coating is almost exactly proportional to theproduct of the current density and the time. At constant current density(which is only approximately reached in plant production), the coatingthickness and consequently the adsorptive capacity for dyes are linearlyproportional to the anodizing time.

It follows that fairly thick coatings are necessary if deep dyeings are to beproduced. A thickness of 12 um anodic coating is usually enough forgeneral application. However, when high color durability is demanded thecoating should be allowed to grow more than 12 um in thickness. Forbuilding parts, a coating thickness of 25 um is necessary.

COLORING

The following dyeing parameters have a pronounced effect on the dyeings

Dyeing temperatureThe rate of dyeing increases with increasing temperature. If thetemperature is too high, buildup of the dye is halted before the full intensityis reached. Dyeings produced at high temperature do not bleed so freely insealing solutions or rinsing waters and also are more difficult to strip. Insome dyes, dyeing at room temperature is possible and easier to control

the desired shade. In most other dyes, the adsorption is very slow causingthe dyeing times over-long. In general, the temperature region of 50- 60Crepresents the optimum compromise between attainable depth andacceptable dyeing time.

Dyeing time


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The normal dyeing time ranges between 3 to 15 minutes. Very short dyetimes of a few seconds may cause many of the dyes to bleed heavily in thesealing solution. Dyeing for longer than 30 minutes brings no advantages.If the required color intensity is not achieved after this time, the problemsmay be the anodic coating being too thin, the dye concentration being toolow or the bath are contaminated by foreign impurities.

pH valueThe optimum dyeing pH normally comes within the region of 5 to 6. Toohigh in pH retards the dye adsorption resulting in pale shade. Too low inpH may cause the aluminum to dissolve into the dye solution causing thedye solution to break down or forms precipitation.

Dye concentrationGenerally, a dye concentration of 3 5 g/l is enough to achieve a deepshade of al colors other than black. 1 3 g/l maybe used for achieve a paleshade. For the black colors, 8 10 g/l must be used to achieve a jet-blacktone.

SEALING

The hot sealing processThe hot sealing closes the pores of the anodic coating and fixes the dyesdeposited in the pores. This process is also referred to hydro-thermalsealing. The hot sealing process is affected by the time and temperaturefactors and by the composition and the pH of the sealing solution. Theminimum sealing time is 2.5 minutes per um. The temperature should be asclose to the boil as possible. The seal solution should be set at pH 5.5 to6.0. The pH adjustment is obtained with dilute acetic acid or sodium

hydroxide.

The cold sealing processThe cold sealing process is only suitable for closing the pores of naturalcolor anodic coating. This type of process is referred to as impregnationsealing. The process depends upon the reaction of an anodic coating withnickel and fluoride ions at room temperature. The minimum sealing time is

about 1.0 minutes per um. The temperature should be as close to 25C aspossible. The seal solution should be set at about pH 6.0.The control ofnickel to fluoride ratio is an important factor in respect of seal quality of thefinal product.

anodizing basics

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