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Everything you need to know about Aluminum Anodizing

What is Anodiz­ing?

Anodiz­ing is an elec­tro­chem­i­cal process that con­verts the met­al sur­face into a dec­o­ra­tive, durable, cor­ro­sion-resis­tant, anod­ic oxide fin­ish. Alu­minum is ide­al­ly suit­ed to anodiz­ing, although oth­er non­fer­rous met­als, such as mag­ne­sium and tita­ni­um, also can be anodized.

The anod­ic oxide struc­ture orig­i­nates from the alu­minum sub­strate and is com­posed entire­ly of alu­minum oxide. This alu­minum oxide is not applied to the sur­face like paint or plat­ing, but is ful­ly inte­grat­ed with the under­ly­ing alu­minum sub­strate, so it can­not chip or peel. It has a high­ly ordered, porous struc­ture that allows for sec­ondary process­es such as col­or­ing and seal­ing.

Anodiz­ing is accom­plished by immers­ing the alu­minum into an acid elec­trolyte bath and pass­ing an elec­tric cur­rent through the medi­um. A cath­ode is mount­ed to the inside of the anodiz­ing tank; the alu­minum acts as an anode, so that oxy­gen ions are released from the elec­trolyte to com­bine with the alu­minum atoms at the sur­face of the part being anodized. Anodiz­ing is, there­fore, a mat­ter of high­ly con­trolled oxi­da­tion the enhance­ment of a nat­u­ral­ly occur­ring phe­nom­e­non.

What Are the Types of Alu­minum Anodiz­ing Process­es?

There are three types of alu­minum anodiz­ing process­es. They are described in more detail below:

1.Type I — Chromic Acid Anodiz­ing

This process uses chromic acid as the elec­trolyte and pro­duces the thinnest coat­ing of all the meth­ods, 2.5 μ (0.0001 in). Despite the reduced thick­ness, this process pro­duces com­pa­ra­ble cor­ro­sion resis­tance to the oth­er two process­es. The coat­ing pro­duced tends to be dark­er and does not accept col­or as well due to its reduced thick­ness and reduced poros­i­ty.

2.Type II — Sul­fu­ric Acid Anodiz­ing

This process makes use of dilute sul­fu­ric acid as the elec­trolyte. It is the most com­mon­ly used tech­nique. A coat­ing with a thick­ness of 5.1 to 15.2 μ (0.0002 – 0.0006 in) can be devel­oped. This coat­ing is hard­er than the one pro­duced by chromic acid anodiz­ing. Parts anodized using the Type II process can eas­i­ly be col­ored with a wide range of dyes. Sul­fu­ric acid is a rel­a­tive­ly low-cost elec­trolyte when com­pared to chromic acid.

3.Type III — Hard­coat Anodiz­ing

This process makes use of sul­fu­ric acid as the elec­trolyte but is used to pro­duce much thick­er coat­ings than Type II anodiz­ing — 12.7 to 76.2 μ (0.0005 to 0.0030 in) due to a high­er volt­age, longer bath immer­sion time and low­er bath tem­per­a­ture. This coat­ing is hard­er than tool steel and is used where high lev­els of wear resis­tance are required. 

The typ­i­cal work­flow of the type II alu­minum anodiz­ing process is as fol­lows.

Rack­ing – Pick­ling – Etch­ing – Chem­i­cal Pol­ish­ing – Anodiz­ing – Col­or­ing – Seal­ing – Dry­ing

Rack­ing: Hang parts on racks. The racks will car­ry elec­tric cur­rent to the parts and will be immersed in acid, alka­li, and Metal­lic salt solu­tions. They are made from tita­ni­um alloy.

Pick­ling: Immerse the parts in an acid or alka­line solu­tion to remove oil, grease, and oth­er impu­ri­ties so they would not affect the anodiz­ing effect.

Etch­ing: Immerse the parts in the acid solu­tion to remove a very thin lay­er on the sur­face, avoid the orig­i­nal oxide lay­er on the sur­face from affect­ing the sub­se­quent anodiz­ing effect, and form a uni­form matt sur­face at the same time. The time of this process will affect the size of the parts, and the etch­ing time of pre­ci­sion alu­minum parts needs to be strict­ly con­trolled.

Chem­i­cal Pol­ish­ing： This process is main­ly used for parts with bright sur­face require­ments. Gen­er­al­ly, the mixed solu­tion of three kinds of anhy­dride is used for chem­i­cal pol­ish­ing. This process can be omit­ted for parts that require a matt sur­face.

Anodiz­ing: The work­pieces are immersed in sul­fu­ric acid solu­tion, and the pore struc­ture alu­minum oxide film is formed on their sur­face by elec­tri­fi­ca­tion.

Col­or­ing: Type II oxi­da­tion gen­er­al­ly adopts dip col­or­ing. The anodized alu­minum parts are immersed in a bath con­tain­ing dye. The dye is adsorbed on the sur­face of the pore open­ing of the anodized film. The col­or pro­duced depends on the type and chem­i­cal prop­er­ties of the dye. Dip col­or­ing is a cheap method that allows man­u­fac­tur­ers to paint var­i­ous col­ors on alu­minum parts. That is why the type II anodiz­ing process is the most pop­u­lar process for alu­minum anodiza­tion.

Seal­ing： After anod­ic oxi­da­tion and dye­ing, the alu­minum parts are immersed in ion­ized water or dis­tilled water to seal the porous struc­ture oxide lay­er. There are three com­mon­ly used seal­ing meth­ods, hot water seal­ing. Add met­al salts such as nick­el, mag­ne­sium, or cobalt to medi­um-tem­per­a­ture water to seal. Seal­ing of room tem­per­a­ture water plus nick­el flu­o­ride and oth­er met­al salts. The oxide lay­er after seal­ing is stronger, smoother, and more durable.

Anodiz­ing Met­al Parts With Pro­to­soon

Anodiz­ing is a wide­ly-used fin­ish­ing process for met­al parts. It can increase a met­al component’s lifes­pan by improv­ing cor­ro­sion resis­tance, wear resis­tance, and more. Anodiza­tion also sim­pli­fies long-term main­te­nance, cre­ates beau­ti­ful and iden­ti­fi­able col­ored com­po­nents, and even pro­motes envi­ron­men­tal­ly friend­ly man­u­fac­tur­ing.

While anodiz­ing your met­al parts is incred­i­bly impor­tant, know­ing how to go about anodiz­ing can be a chal­lenge. Pro­to­soon, a sea­soned man­u­fac­tur­ing part­ner, can help you achieve the best anodized fin­ish for your com­po­nents. Con­tact us today to get start­ed.

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