Longsheng

Two "blacks", two completely different fates

If you've handledaluminum parts, you've probably seen it. You just got a shiny new part and chose the low-cost "black" treatment, expecting a perfect matte black effect. At first, it looks good. But a few months later, during use or cleaning, you find that the surface begins to show annoying scratches, or even worse - the paint film begins to lift and peel from the edges or corners. Time freezes in an instant, followed by frustration and depreciation of product value - either spending energy on rework and repair, or worse, directly facing customer complaints or part scrapping.

This scene of quality collapse is what every manufacturer or user who pursues professionalism and durability tries to avoid. But the fact is that mostsurface treatmentfailures are not due to bad luck, but to a fundamental misunderstanding: confusing the essential difference between "coating" and "conversion" to achieve "black" - that is, "black anodized paint" and true "black anodizing".

The good news is that this can be avoided. Understanding the core principles and differences between the twoprocesses of "painting" and "anodizing" is a key step in ensuring that your parts get a truly long-lasting, wear-resistant, and reliable matte black.

To save you time and avoid future losses, let's get to the heart of it right away.

A quick comparison chart of the differences between black anodized paint and black anodized

The difference between them is not as simple as price, but goes deep into the core level of essence, process, durability and applicability:

After the bitter experience in 2019, we set an iron rule:

"All aluminum molds must undergo Type III hard anodizing treatment- this is not a question of choice, but a matter of customer investment life-and-death protection."

What you are receiving, apart from knowledge, is the "failure insurance" of LS molds

This manual combines our:

7-year database of aluminum mold surface treatment failure analysis
142 original records of process comparison experiments
23 quality monitoring reports of cooperative injection molding factories

"Once you understand why the hard anodizing's microporous structure will hold back the black dye, once you understand how the spray paint coating flakes off the substrate in the thermal cycle - you haven't just got the technology mastered, but the basic decision-making powers to avoid six-digit losses."

What is True Black Anodizing?

"Anodizing isn't 'applying' something, but making aluminum 'grow' a solid protective covering." That sentence describes the process of anodizing exactly - it is an electrochemical conversion process and not merely a coating. True black anodizing is based on this "growth" process, and deep, long-lasting black color is built-in permanently into this "protective shell" by a specific process.

Fundamental principles and process details:

1. Preparation (Preparation - Laying the foundation):

(1) Purpose: Clean the surface of the aluminum piece absolutely, grease-free, oxide- or impurity-free. This is the state required for the formation of a homogeneous, good-quality oxide layer.

(2) Process:

Chemical degreasing/degreasing: Use alkaline or acidic solutions to degrease the organic contaminants such as processing grease, fingerprints, etc.

Pickling/alkaline etching: Remove naturally formed oxide films and minor surface defects to form an even, activated surface. Usual agents used were nitric acid, sulfuric acid or sodium hydroxide solutions.

Neutralization/deashing: Remove any remaining chemicals (e.g. black ash) from pickling/alkaline etching, usually with nitric acid or a trade-neutralizer.

Water washing: Wash cleanly with deionized water preferably after each step to prevent cross-contamination between chemicals.

2. Conversion/Growth (Core Electrochemical Process):

(1) Objective: To create a thick, rigid, porous anodic aluminum oxide film in situ on the aluminum substrate surface.

(2) Method:

A thoroughly cleanedaluminum pieceis used as the anode (positive electrode) and is placed in a low-temperature (usually 15-22°C) acidic electrolyte (sulfuric acid being the most popular, but oxalic acid or mixed acids can also be employed).

A cathode (negative electrode, usually lead plate or stainless steel) is introduced into the electrolytic cell.

Direct current is used.

(3) Reaction:

The current induces the aluminum atoms on the aluminum anode (workpiece) surface to experience an oxidation reaction: 2Al + 3H₂O → Al₂O₃ + 6H⁺ + 6e⁻.

Concurrently, a hydrogen evolution reaction at the cathode: 2H⁺ + 2e⁻ → H₂.

(4) Result:

A film of amorphous aluminum oxide (Al₂O₃) is created on the aluminum surface.

Thisaluminum oxide layer possessesa distinctive honeycomb porous structure. Think of it as having a tiny, very neat and organized coral reef forest on the metal surface. Every "coral polyp" (unit cell of aluminum oxide) possesses a micropore in the center, which is perpendicular to the surface.

Oxide layer thickness is precisely controlled by oxidation time, electrolyte concentration, temperature and current density. Black anodizing usually has to be left to reach a certain thickness (e.g., over 10μm) in order to achieve color depth and hardness.

3. Dyeing (Injecting Black):

(1) Purpose: Inject black dye molecules into the open microporous structure of the oxide layer.

(2) Process:

Rigorously rinse the anodized aluminum component (now containing many open micropores).

Soak in a dyeing tank. To achieve true black, two main dyes are used:

Organic dyes: The most common one, having numerous types, producing a deep black. The particles of the dye penetrate and adsorb onto pore walls by physical adsorption or loose chemical bonds. Dyeing time, temperature, concentration and pH value must be strictly controlled to ensure color uniformity and intensity.

Inorganic salt dyeing (e.g., tin/nickel salts): Colour is accumulated by precipitation of metal salts on the pore floor. Some processes (e.g., "two-step" black) can provide very deep, heat-stable and light-stable black, but the process is more complicated and the colours are limited in their range of choice.

(3) Key: The color effect strongly depends on the properties of the previously created oxide layer (uniformity, porosity). Real black requires that the dye penetrates uniformly across the entire porous depth.

4. Sealing (Sealing - Locking Color and Performance):

(1) Purpose: To seal micropores on the surface of the oxide layer, permanently set the dye in position, and greatly increase the corrosion resistance, wear resistance and life of the oxide layer. This is the most critical process to achieve "true" black anodizing. If not sealed, color will easily peel off or wash away.

(2) Process and Principle:

Hot water sealing: The earliest method. Workpiece, blackened, is immersed in deionized water at boiling point (95-100°C) or hot water containing additives (e.g., nickel salts, fluorides). Hydration swelling of the aluminum oxide layer and thickening at elevated temperature (Al₂O₃ + H₂O → Al₂O₃·H₂O boehmite) increases its volume physically closing off pores.

Cold sealing: Use room temperature or medium temperature solutions of nickel fluorides and salts. The pores are sealed by nickel/fluoride ions deposited in the pores or by induction of hydration reactions. It is low energy intensive and eco-friendly, but slower.

Medium temperature sealing: It is a blend of heat sealing and cold sealing.

(3) Results

Permanent color lock: The color molecules are sealed solid inside the pores and cannot migrate out or be washed away easily.

Increased protection: The sealed oxide layer becomes a tight barrier, significantly improving corrosion resistance (e.g., passing salt spray tests), wear resistance and anti-pollution properties.

Stabilize the surface: Reduce surface adsorption and provide appearance stability.

Real black anodizing is not only spraying or electroplating. It is a robust integrated process sequence of electrochemistry-physical-chemistry: an in situ grown porous aluminum oxide "skeleton" in the aluminum matrix, and the black dye then filled deep into its pores, and sealed color and performance forever by closing pores. Its core value is the deep integration of color with matrix and the derived high durability, protection and appearance. Understanding the essence of "growth" rather than "addition" is the key to understanding this process.

What is Black Anodized Paint?

1. Misleading terminology

The name "black anodized paint" can easily lead people to believe that it is achieved through anodizing, but in fact it is just a coating that imitates the appearance of anodizing. Anodizing is the process of electrochemically generating an oxide film on the surface of metal (such as aluminum), while "anodized paint" simply uses coatings to simulate similar visual and tactile effects. The market may use this term to borrow the high-end industrial image of anodizing, but consumers need to be clear: it is essentially paint and is fundamentally different from real anodizing.

2. Core principles and formula characteristics

The specialness of this coating lies in its formula design, which usually has the following characteristics:

Matte/satin texture: By adding matting agents or special resins, the low reflectivity surface of anodizing is imitated.

Black tone stability: Weather-resistant pigments (such as carbon black) may be used to avoid fading.

Adhesion enhancement: Add coupling agents or primer components to ensure the bonding of the coating to the metal substrate (such as aluminum and steel).

3. Process flow (compared with real anodizing)