Technical Notes

Hard Anodizing

Principle

Anodizing is an electrochemical process in which an aluminum part is connected as the anode and current is passed through a sulfuric acid electrolyte to form an aluminum oxide coating on the surface. Unlike plating, the coating is not a layer placed on top; the material surface itself is converted to oxide, so it does not peel off easily. The coating improves corrosion resistance, wear resistance, hardness and electrical insulation, and gives the surface a consistent appearance.

The coating consists of a thin, dense barrier layer in contact with the base material and a porous layer above it with regularly arranged fine pores. Because the oxide conducts electricity poorly, the current at a given voltage decreases as the coating thickens, and this current governs both the growth and the dissolution of the coating. For this reason, the stability of the current supplied by the rectifier determines the uniformity of coating thickness and the surface roughness.

The hardness of a hard coating depends on how far coating dissolution is suppressed. At the pore base, coating growth and dissolution by the electrolyte occur at the same time: too much dissolution widens the pores and softens the outer layer, lowering hardness, while too little leaves the pore base unopened, thickening the barrier layer until the voltage climbs and local overheating (burning) occurs. Hard anodizing uses the strongly dissolving sulfuric acid electrolyte at a lowered electrolyte temperature so that dissolution is held just below the point where burning occurs, and grows a thick, dense coating in that state. This is why electrolyte temperature and a stable current supply are the key control items for hard coating quality.

Properties of Hard Coatings

Hard anodizing runs at higher current and voltage and at a lower electrolyte temperature than decorative and protective (soft) anodizing, producing a thicker, denser coating. As a result, wear resistance and hardness increase, and the edge radius (R) and thickness tolerance are controlled together to the drawing. In standards, MIL-PRF-8625 Type III corresponds to hard coatings.

Thickness and hardness do not always move in the same direction. Coating thickness increases with current density and the current application method, but if growth is too fast or the pores become large, the coating structure loosens and hardness can drop. Hard coatings therefore treat thickness and surface hardness as separate inspection items, each controlled to the value specified on the drawing.

As the coating grows, part dimensions change as well. For parts with dimensional tolerances, dimensional growth from the coating is included as an inspection item and checked together with the drawing dimensions. Some drawings prohibit machining or post-treatment after anodizing, so the process sequence follows the drawing instructions.

Applications

In braking systems, EF Technology applies hard coatings to piston and simulator components subject to reciprocating friction. These components require surface hardness, coating thickness, white rust rate after salt spray, and properties after brake fluid contact as specified requirements.

In HVAC and thermal management, EF Technology has applied hard coatings to components with friction, such as the sliding contact areas of compressors. For robotics components, development processing is under way with chemical conversion coating and soft anodizing, and hard coatings are also under review for drive components with repeated friction.

Related Standards (Supported)

Hard coatings meet Hyundai/Kia MS633-06 (functional anodizing), ZF (TRW) TS2-25-102 (Hard anodising for aluminium alloys), MIL-PRF-8625 Type III and Mando MES-M-00-SF-009. Each standard differs in coating thickness measurement locations, hardness measurement method (surface/cross-section), salt spray duration and acceptance criteria, and whether sealing is required, so the standard and grade on the drawing are checked first.

Inspection Items

Coating thickness is measured by location with a non-destructive thickness gauge and, when needed, directly on a cross-section after the sample is cut, mounted and polished. Surface hardness is checked with a Vickers hardness tester and corrosion resistance with a salt spray tester. For parts that specify sealing, sealing quality is checked by a sealing test or the mass loss test defined in the standard, and coating weight is measured by the stripping method (ASTM B137).

Soft Anodizing

Principle and Properties

Soft (decorative and protective) anodizing runs at lower current and voltage than hard anodizing and at an electrolyte temperature near room temperature, producing a coating with a regular porous structure. The coating is thinner than a hard coating, but the surface is smooth and the color is even, and sealing greatly increases corrosion resistance. In MIL-PRF-8625, sulfuric acid anodizing is Type II, undyed coatings are Class 1 and dyed coatings are Class 2.

Soft coating quality is judged by thickness uniformity and surface roughness. The R&D department compared coating thickness and roughness on die-cast test pieces while changing the rectification method and electrolysis conditions. A semiconductor-switching rectifier produced lower thickness variation and roughness than a diode rectifier, and electrolyte temperature and concentration affected thickness more than roughness. Based on these results, EF Technology has set the range of conditions for a uniform coating.

Applications

EF Technology applies soft coatings to components where corrosion resistance, appearance and the dimensional stability of assembly and sealing surfaces matter, such as hydrogen tank connection parts, HVAC valves and motor housings. For motor components, black-dyed coatings are applied to exterior parts and undyed coatings to shafts and nuts, based on MIL-PRF-8625 Type II.

Blocks machined after forging have both machined and forged surfaces on the same part. A cross-section comparison of coating thickness on both surfaces of the same sample showed equal thickness; the forged surface looked different only because the base material surface was less smooth. Parts that combine areas with different surface conditions are handled by checking the coating area by area in this way.

Related Standards (Supported)

Soft coatings meet MIL-PRF-8625 Type II (Class 1 and 2). When the customer drawing specifies its own standard or grade (for example, a grade such as AA-DE that specifies thickness, sealing, adhesion and salt spray duration together), that standard takes priority. Drawings for die-cast and forged parts sometimes specify only a thickness range and measurement method without a standard number; the measurement locations and thickness range on the drawing then become the control criteria as written.

Inspection Items

Coating thickness is measured non-destructively at the locations specified on the drawing, and appearance — color variation, stains and untreated areas — is recorded visually and with photographs. For dyed coatings, color is quantified with a colorimeter (L*a*b*, ΔE) when needed, and color loss and surface change after sealing are checked. Corrosion resistance, hardness and dimensions are checked by salt spray, hardness and dimensional growth measurement when the drawing requires them.

Pretreatment

Why It Matters

An anodic coating takes on the surface condition of the base material as it is. If machining oil, rust-preventive oil, die-casting release agent residue or alloy residue left after etching remains on the surface, the coating grows unevenly over it, and the unevenness shows through the transparent coating as stains. Published studies confirm that different pretreatment methods change the composition of the surface oxide film and the enrichment of alloying elements, which in turn changes the composition and corrosion behavior of the coating formed afterward.

Role of Each Step

Degreasing removes cutting oil, rust-preventive oil and contamination; after cleaning, a water-break check confirms that water spreads evenly over the entire surface to verify repeatability. Etching and surface conditioning remove the natural oxide film and machining marks, but they affect appearance and dimensions; for parts with machined surfaces or dimension-sensitive areas, whether to apply them and to what degree is determined by testing and kept to a minimum. Desmutting (neutralizing) removes Si, Cu, Fe and other alloy residues left on the surface after the aluminum dissolves, creating a uniformly active surface.

Rinsing prevents chemical carry-over and re-oxidation between process steps, and rinse water condition and conductivity are recorded and controlled. Racking fixes the electrical contact position by hanging parts on dedicated racks; because contact position and current distribution can cause local differences in coating thickness and stains, contact points, layout and part orientation are recorded together.

Pretreatment Designed to Protect Functional Areas

Strong alkaline etching selectively dissolves only the aluminum matrix, which concentrates impurities such as Cu and Si on the surface instead, and it brings dimensional loss and rougher surfaces. Fluoride-based etching can improve appearance, but die castings have different solidification structures by location, so appearance variation remains after etching, and the roughness, dimensions and alloy composition of functional areas change. For parts that include functional areas such as bores, sealing surfaces and fastening surfaces, EF Technology therefore gives priority to functional requirements, builds pretreatment mainly around mild degreasing, and agrees appearance criteria with the customer.

Plasma Pretreatment Research

Conventional chemical pretreatment adds process cost and management workload through chemical dosing, concentration control, cleaning and wastewater treatment, and its immersion batch operation is difficult to link with continuous production lines. The R&D department has studied a non-contact pretreatment that cleans and activates metal surfaces with a plasma jet operating at atmospheric pressure, without a vacuum chamber. The goal is to remove organic contamination and activate the surface with the ions, radicals and ultraviolet light generated by the plasma, reducing chemical use and wastewater while enabling continuous processing.

The research has resulted in three registered patents. The first patent (No. 10-2922025, Pretreatment method for metal materials using plasma) claims a seven-step process — material preparation, mounting on the transfer device, powering the plasma generator, supplying surface modification gas, atmospheric-pressure plasma jet discharge, driving the transfer device and passing through the treatment zone — and a generator structure consisting of a gas chamber, gas delivery body, electrode mounting body, spray body and spacer.

The second patent (No. 10-2878394, Pretreatment method for metal materials in which workpieces circulating along a track are treated by a plasma generator) covers the transfer method. Toothed planets engage between a fixed track and a circulating track that runs outside it, so that when the circulating track turns, each planet rotates on its own axis while revolving along the fixed track. Parts hang below the planets and rotate and revolve with them, so the entire outer surface, which a spray from a single direction cannot reach, is evenly exposed to the plasma.

The third patent (No. 10-2941480, Pretreatment method for metal materials using plasma in which workpieces are hung on hanging members and circulated) covers how parts are hung. A pair of cross-linked hanging members opens and closes like scissors around a hanging shaft to hold parts of various shapes, an insert member fixes the angle between them, and the hanging members themselves fit into and detach from the planet disc. Operators load and unload parts without entering the treatment zone, and product changeover and maintenance become easier.

The plasma generation module is a stack of a gas chamber that receives and diffuses the supply gas, a gas delivery body that distributes the gas evenly, a first electrode assembly that applies high voltage to create the discharge, a second electrode assembly wrapped in a dielectric with multiple discharge holes, and a spray body that ejects the plasma and cools the electrodes; spacers of different thicknesses adjust the electrode gap. Concept design of each component based on the patent drawings is complete. Evaluation methods for cleaning and activation effect, uniformity, and reduction of chemicals, water and wastewater are defined, and these effects will be confirmed in prototype verification.

Jigs and Masking

Parts that require partial treatment need a jig that sets the electrical contact position while covering the areas not to be treated. Registered Patent No. 10-2449453 (Plating jig device with a simple marking function) continuously fixes jig bars to a current-carrying center bar, holds parts in a fixing section formed by bending the end of each jig bar, and fits a cylindrical marking member onto a fixing pin to block the groove in the non-treated area. The marking member has a two-step guide with different diameters, so it covers non-treated areas of different sizes without changing the member, and its simple structure is an advantage in fabrication and work speed.

Material Characteristics

How Alloying Elements React with Process Solutions

The alloy composition and temper of the material have the greatest influence on anodizing results. Aluminum and magnesium oxidize in the electrolyte and form a transparent, uniform coating, whereas silicon, copper, iron and zinc either do not take part in the oxidation or dissolve, changing the color, hardness and uniformity of the coating. The figure below summarizes how each element behaves in the pretreatment solutions and the sulfuric acid electrolyte, and what it leaves in the coating.

Silicon does not dissolve in aluminum, exists as crystals and does not oxidize, so it remains in the coating, appears as gray or black particles and lowers coating density. Copper raises the interface resistance during electrolysis, causing local heating (burning), and makes the coating thinner, softer and reddish. Iron exists as intermetallic compounds that reduce brightness, which is why low-iron 6063 and 6463 anodize to a clearer silver than 6061. In zinc-rich 7xxx alloys, intermetallic compounds affect coating growth and local dissolution, which can produce a mottled gray, blue-gray or brown-black appearance and color variation; even within one alloy, the coating structure changes with temper.

Alloying elements also show their influence in pretreatment. Alkaline etching dissolves only aluminum and leaves silicon, copper and iron on the surface as smut, which must be removed in the desmut step so that the coating forms uniformly during anodizing.

Comparison by Alloy

Even under identical conditions, each alloy family differs in shade, hardness and maximum thickness; in particular, the more copper and silicon, the thinner, softer and darker the coating. The figures in the table are values from published standards and association references, intended as a design reference rather than a guarantee for individual products.

Alloy Limits Set by the Specifications

MIL-PRF-8625 does not apply hard anodizing (Type III) to alloys containing more than 5 % copper or more than 8 % silicon; alloys with more than 8 % silicon are processed only with purchaser approval. Heat-treatable alloys must be in a fully heat-treated condition such as T4, T6 or T73, and the wear acceptance criterion is relaxed for alloys with 2 % or more copper (all 2xxx, 7050, 7178). Riken Alumite also states that its standard performance guarantees do not apply to alloys with 5 % or more copper or 8 % or more silicon.

Natural Color by Material

An undyed anodic coating is nearly transparent, so the structure and alloying elements of the base material show through as color. Low-alloy 5052 and 6061 series anodize to a bright, uniform color, and discoloration and non-uniformity increase with alloying element content. Al-Si die-casting alloys such as ADC12 take on a gray base from Si with a reddish-brown tone from Cu and Fe, and because the eutectic Si particles barely take part in the oxidation, the coating grows thin and uneven. High-copper 2xxx alloys tend to give lower hard-coating hardness, so even hard anodizing specifications set separate hardness criteria for them. These color differences are material characteristics rather than functional defects, so color criteria are agreed in advance with limit samples and the material lot, temper and machining direction are recorded together.

Post-treatment: Sealing and Color

Role of Sealing

A sulfuric acid anodic coating has a porous structure with regularly arranged fine pores, so without further treatment the pores become paths for corrosive substances to penetrate. Sealing is a post-treatment that closes these pores by chemical reaction after anodizing; it greatly improves corrosion resistance and makes the surface easier to keep clean. In hot water sealing, aluminum oxide reacts with water and converts to boehmite (AlO(OH)), expanding in volume to close the pores; in sealing with nickel salts, nickel hydroxide also forms and closes the pores. Low-temperature sealing near room temperature uses less energy and is also used in industry.

Whether to seal is determined by the standard and the drawing. Hard coating standards basically do not require sealing or allow post-treatment only when specified on the drawing, whereas functional anodizing standards make sealing mandatory and judge sealing quality by mass loss. Dyed coatings must be sealed to keep the dye from leaching out, so the sequence of sealing after dyeing and final rinsing is standardized.

Sealing Verification

Sealing condition is checked by the test methods of KS D 8315 (Test methods for sealing quality of anodic oxide coatings), and some standards judge it by mass loss after acid immersion. When the standard requires, corrosion resistance is also checked by salt spray test.

Anodic coatings "grow half in, half out" — growth mechanism and dimensional change

1.1 Difference from plating: the base metal itself converts to oxide

Anodizing connects the part as the anode (+) and passes current through a sulfuric acid electrolyte, converting the aluminum surface into aluminum oxide (Al₂O₃). Unlike plating, the process does not deposit metal on the surface. Existing aluminum atoms combine with oxygen to form oxide, so roughly half of the coating forms below the original surface line (where base metal is consumed) and the rest forms above it (by the increase in volume). On the shop floor this appears as "half eroded, half grown," and the U.S. military specification MIL-PRF-8625 also states that each surface increases in dimension by about one half of the coating thickness.

The electrode reactions can be written simply as follows. At the anode (part), 2Al + 3H₂O → Al₂O₃ + 6H⁺ + 6e⁻; at the cathode, 2H⁺ + 2e⁻ → H₂, releasing hydrogen gas. Oxide forms at the metal/oxide interface, where aluminum ions meet oxygen ions from water. At the same time, sulfuric acid gradually dissolves the pore bottoms, so the coating is not a fully dense layer but a porous structure with countless fine pores.

The volume expansion ratio when metal converts to oxide (Pilling–Bedworth ratio) is reported in the range of 1.26–1.67 in measured sulfuric acid anodizing and increases with current density. This volume expansion produces a coating thicker than the aluminum consumed, and the difference is the portion that grows outward from the surface line. The exact inward/outward ratio varies with alloy, temper and process conditions, so the specifications use the expression "approximately one half."

1.2 Coating structure: barrier layer and porous layer

The coating consists of two layers. At the bottom, in contact with the base metal, is a very thin barrier layer with no pores. Above it, a porous layer, in which each hexagonal cell has a single pore at its center, makes up most of the thickness. Pore diameter is approximately 10–150 nm and is set mainly by the electrolyte; conventional sulfuric acid anodizing gives the smallest pores, about 15–25 nm. Under the same sulfuric acid conditions, raising the voltage leaves the pores themselves about the same size but widens the spacing between them. For example, a pore spacing of about 35–40 nm at 15 V becomes about 45–50 nm at 20 V. The holes stay the same size but there are fewer of them in the same area, so at higher voltage the pores take up a smaller share of the coating (lower porosity) and the coating is denser. These pores allow dye to penetrate for coloring; without sealing, contamination and stains occur easily.

What actually passes through the coating inside a cell is ions, not electrons. Aluminum oxide is an insulator, so current flows as ions move under the strong electric field across the thin barrier layer. Oxygen ions (O²⁻, OH⁻) move inward from the electrolyte and form new oxide at the metal/oxide interface, while aluminum ions (Al³⁺) move outward; some become coating and some pass into the electrolyte at the pore base. In the barrier layer at the pore base, coating growth and field-assisted dissolution take place at the same time, and the barrier layer settles at the thickness where the two rates are equal. Barrier thickness and cell diameter are proportional to the applied voltage, so a higher-voltage process produces larger cells with thicker walls.

The electrolyte changes coating properties through this balance. Sulfuric acid is a strong acid that dissolves the oxide readily, so equilibrium is reached with a thin barrier layer at low voltage; oxalic acid dissolves it weakly, so equilibrium is reached with a thick barrier layer at high voltage. As a result, sulfuric acid coatings incorporate a large amount of electrolyte anions (sulfate) and are relatively soft but dye well, whereas oxalic acid coatings are closer to pure alumina, harder and more insulating, and take on a self-color in the yellow to bronze range depending on the alloy. Sulfuric acid is the standard electrolyte for both the soft (Type II) and hard (Type III) specifications; its high conductivity makes heat management easier and its cost is low, which is why it is widely used for automotive mass production. EF Technology processes both soft and hard anodizing in sulfuric acid.

1.3 The inward/outward ratio differs between soft (Type II) and hard (Type III)

Hard anodizing (MIL-PRF-8625 Type III) is generally taken as 50 % inward / 50 % outward, and supplier data give more detailed values such as 55 % penetration / 45 % growth (approx. 0.0009 in growth per surface for a 0.002 in coating). Conventional sulfuric acid anodizing (Type II) produces a thinner coating with relatively greater dissolution by the electrolyte, so many sources cite approx. 2/3 penetration and 1/3 growth; field experience has reported up to 80 % penetration / 20 % growth on 6061. It is therefore safe to assume a dimensional increase of 1/3–1/2 of the coating thickness for soft anodizing and approx. 1/2 for hard anodizing.

1.4 Calculating dimensions: +t/2 per surface, +t on diameter

With coating thickness t, a flat surface grows by approx. t/2 per side, and a shaft (outside diameter) grows on both sides, so its diameter increases by approx. t. A hole (inside diameter) decreases in diameter by approx. t. For example, a 50 µm (0.002 in) hard coating results in approx. +50 µm on a shaft diameter and approx. −50 µm on a hole diameter. MIL-PRF-8625 gives the example of a 50 µm (0.002 in) pre-machining allowance per surface for a 100 µm (0.004 in) coating.

Threads change more. The major and minor diameters of a thread increase by twice the growth, and the pitch diameter of a 60° thread increases by four times the growth. This is because the coating grows on both flanks simultaneously; applying a hard coating directly to threads often prevents Class 2 fit threads from assembling. The drawing should therefore specify one of the following for threads: masking, tapping after anodizing, or oversize taps.

1.5 Tolerance design guidelines

The specification recommends stating both the "machined dimension" and the "finished dimension after coating" on the drawing. At interference and transition fits, the coating is hard like a ceramic but brittle, and press-fitting can damage or peel the coating. If required, a hard coating can be built up thicker and then ground or lapped to size; soft (Type II) and chromic (Type I) coatings are too thin for this method.

Small holes and tapped holes have poor electrolyte circulation, so the coating varies widely from "none" to "normal thickness." MIL-PRF-8625 advises that only holes of 1/4 in (approx. 6.35 mm) diameter or larger be considered normally anodized. The specification also notes that very tight dimensions with a total tolerance of 0.0004 in (approx. 10 µm) or less can be finished by honing or lapping after anodizing, with those surfaces supplemented by chemical conversion coating or similar treatment.

1.6 Thicker hard coatings require larger edge radii

The coating grows perpendicular to the surface, so at sharp edges the coatings growing from the two faces cannot meet, leaving a void where the coating may be discontinuous. MIL-PRF-8625 provides a table of recommended radii by coating thickness: approx. R0.8 mm (1/32 in) for 25 µm (0.001 in), approx. R1.6 mm (1/16 in) for 50 µm (0.002 in), approx. R2.4 mm (3/32 in) for 75 µm (0.003 in), and approx. R3.2 mm (1/8 in) for 100 µm (0.004 in). The AAC (Aluminum Anodizers Council) also recommends approx. 1/16 in R per 1 mil of coating, and Riken Alumite of Japan specifies minimums of R0.2 for 25 µm and R0.3 for 50 µm. A chamfer (C) alone creates new sharp edges, so the specification states that edges must be rounded even after chamfering.

The standard hard coating thickness under MIL-PRF-8625 is 50 µm (0.002 in), with a tolerance of ±20 % up to 50 µm and ±10 µm (±0.0004 in) above that. Typical thickness ranges given by the specification are 1.8–25 µm for Type II and 12.7–114 µm for Type III. However, wear resistance decreases as thickness approaches 75 µm (3 mil), so "thicker is better" does not hold. The top layer of a hard coating is less dense and rougher than the bottom layer; the specification describes building up precision parts thicker and finishing them by lapping.

Dimensional and surface changes in pretreatment

2.1 Why degreasing, etching, desmutting and rinsing are all required

After machining, part surfaces carry cutting oil, fingerprints, marking ink, natural oxide film and intermetallic compounds of alloying elements (silicon, copper, iron, etc.). Anodizing converts only pure aluminum into oxide, so the purpose of pretreatment is to make the exposed aluminum as clean and uniform as possible. The specification requires parts to be in a water-break free condition, completely free of oil, paint, welding flux and other foreign matter.

Degreasing (alkaline cleaning) removes oil and contamination and removes almost no base metal. Etching (with alkalis such as caustic soda) dissolves a thin layer of aluminum to remove the natural oxide film and machining marks, producing a matte surface. Base metal is actually removed in this step, so dimensions decrease slightly. The amount removed varies widely with conditions, so precision parts should be specified as "non-etch" or "light etch."

Desmutting uses acid to remove the black, powder-like residue (smut) left on the surface after etching. Alkaline etching dissolves only aluminum and cannot dissolve alloying elements such as copper, silicon and iron, which remain on the surface as smut. Nitric acid-based desmutters remove copper residue, and fluoride-containing desmutters remove silicon. Omitting this step results in stained or cloudy gray-brown coatings and poor adhesion.

Rinsing is required between every step and prevents solution from one step from being carried into the next tank. Insufficient rinsing allows alkali or acid trapped in tapped holes and crevices to seep out after anodizing, forming white powder or streak stains (bleed-out), and contaminates the sealing tank, causing sealing defects. Final rinsing with deionized water is particularly important for preventing stains.

2.2 Points to address at the machining stage

Polishing aluminum with iron-containing abrasives (steel wool, iron oxide polishing compounds, steel wire brushes) embeds iron particles in the surface, which appear as gray spots or pits after anodizing; the specification prohibits their use. For the same reason, belts and grinding wheels dedicated to aluminum should be used, and tools used on other metals should not be shared. Adhesive tape residue, marking ink and sticker adhesive are not readily removed by normal pretreatment and can remain as stains.

Anodizing does not conceal surface defects; it reveals them. Machining marks, scratches, burrs and embedded chips become more visible after etching, and chips in tapped holes trap solution. The specification therefore requires anodizing after all heat treatment, machining, welding, forming and perforating are complete. Deburring, edge radiusing and removal of chips from holes must be completed by the machining supplier before parts are shipped to us.

Frequently asked questions

Thermal properties are also considered. The thermal conductivity of the coating is 1/10–1/30 that of aluminum and its emissivity rises to approx. 0.8. Hard coatings can reduce fatigue strength, so thickness must be set carefully for parts under cyclic loading.

Drawing callout guide

5.1 MIL-PRF-8625 Types and Classes

MIL-PRF-8625 (formerly MIL-A-8625) classifies electrolyte and process by Type and coloring by Class. Type I and IB are chromic acid, Type IC is a non-chromate alternative, Type II is sulfuric acid (soft), Type IIB is thin-film sulfuric acid, and Type III is hard. Class 1 is non-dyed and Class 2 is dyed. An example callout is "MIL-PRF-8625 Type III, Class 2, Black, thickness 50 µm (0.002 in)."

The specification requires purchase documents to state, in addition to Type and Class, the Type III thickness, any special sealing requirements, dye color and uniformity, contact mark locations, and acceptable appearance variation due to material differences such as welds and cast surfaces. If thickness is not stated on the drawing, Type III is processed to the default 50 µm (0.002 in), and Type II is governed by coating weight (minimum 1000 mg/ft²) rather than thickness.

5.2 ISO 7599 / ISO 10074 and AA thickness classes

ISO 7599 (decorative and protective) designates coatings by thickness class "AA + number," where the number is the minimum average thickness (µm). AA5, AA10, AA15, AA20 and AA25 are typical classes, and the minimum local thickness is 80 % of the average (20 µm for AA25). Areas within 5 mm of contact points are excluded from thickness measurement, and hard coatings are covered separately by ISO 10074. An example callout is "ISO 7599 - AA20." JIS H 8601 and KS D 8301 are based on ISO 7599 and use the same AA class designations.

5.3 Aluminum Association (AA) finish designations

Architectural and decorative applications use Aluminum Association designations such as "AA-M12C22A41." M denotes mechanical finish, C chemical finish and A anodic coating. A2x is protective/decorative under 0.4 mil (10 µm), A3x is architectural Class II at 0.4–0.7 mil (10–18 µm), and A4x is architectural Class I at 0.7 mil (18 µm) or more. The final digit 1 is clear, 2 integral color, 3 dyed and 4 electrolytically colored. Where an automaker specification is designated (e.g., Hyundai/Kia MS 633 series), the class designations and test items of that specification must be transferred to the drawing as written.

5.4 Thickness callout: distinguish "coating thickness" from "dimensional change"

The thickness on the drawing is the total coating thickness; the dimensional change is half of that (per surface). For example, "hard 50 µm" means +50 µm on a shaft diameter and +25 µm per surface, so, as the specification recommends, the drawing should state both the machined dimension and the dimension after coating, or at minimum indicate which is the finished dimension. Thickness is measured as the average of 8 or more points by eddy current (ASTM B244 / ISO 2360), with cross-section microscopy (ASTM B487 / ISO 1463) as the referee method in case of dispute.

5.5 Masking, contact and non-treated area instructions

Areas that must remain uncoated, such as threads, precision fits and electrical ground surfaces, are marked "no anodizing (mask)" on the drawing, with the extent dimensioned. Acceptable contact mark locations, sealing (unsealed is the default for wear-resistant hard coatings) and color standards (color sample or color difference limit) must also be stated on the drawing or purchase order to become specification requirements. The specification calls for significant surfaces to be marked on the drawing; if they are not marked, the processor decides according to normal practice.