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What is passivation?

Passivation refers to the process of forming a dense, stable, and inert oxide film on the surface of a metal through chemical or electrochemical means to improve its corrosion resistance. It is a set of procedures that transition from an "active dissolved state" to a thermodynamically stable state. Different from coatings: no foreign substances are added, activating the metal's own corrosion resistance potential.

The role of passivation

Strengthening corrosion resistance, especially for stainless steel, passivation is a key step in achieving superior corrosion resistance. It can effectively prevent pitting and rusting. Remove iron pins, welding slag, grease, coolant, etc. introduced during the processing. This process is mainly used for stainless steel, aluminum alloy, copper alloy, etc.

The principle of passivation

When parts are placed in a specific passivation solution, atoms on the metal surface react with oxidants in the passivation solution. For example, in iron-based alloy parts, iron atoms lose electrons and are oxidized, while certain components in the passivation solution form a stable oxide structure on their surface. This oxide film is very dense, with a thickness generally ranging from nanometers to micrometers. It can prevent oxygen, moisture, and other corrosive substances from further contacting the metal atoms on the surface of the parts, thereby achieving the goal of preventing rust and corrosion of the parts. Moreover, this oxide film also has a certain degree of stability, which can maintain its protective performance for a long time. The thickness of the passivation film is usually ≤ 1 μ m, transparent or slightly rainbow colored. The protective effect of passivation film depends on chemical stability and self-healing ability, rather than physical thickness. Passivation treatment does not change the color, size, or mechanical properties of the workpiece.

Classification of passivation processes

The most commonly used passivation processes in industry can be divided into two categories: chemical passivation and electrochemical passivation, each with its own characteristics due to differences in substrate (stainless steel, aluminum alloy, copper alloy, etc.) and solution system (chromate, chromium free, acidic, alkaline, etc.).

Chemical passivation

Chemical passivation is a process that involves immersing metals in a solution containing oxidants and film-forming agents, and spontaneously forming a passivation film through chemical reactions. It is easy to operate and has low cost, making it the "first choice" for hardware, home appliances, and ordinary mechanical parts. Taking stainless steel as an example, the most classic formula is the nitric acid+hydrofluoric acid system (or pure nitric acid system). Nitric acid, as a strong oxidant, promotes the preferential oxidation of chromium element to form Cr ₂ O3 film; Hydrofluoric acid acts as an "etching" agent, removing free iron from the surface (iron in stainless steel can easily become a corrosion starting point if exposed). Taking a certain stainless steel kitchenware product as an example, when the nitric acid concentration is below 20%, the film layer is thin and uneven, and the workpiece rusts in less than 48 hours through salt spray testing (a common method for detecting corrosion resistance); When the concentration exceeds 30%, the corrosion of the metal by the solution intensifies, and the film layer is easily "dissolved", resulting in a decrease in salt spray time. Finally adjusted to 25% nitric acid+3% hydrofluoric acid, temperature controlled at around 45 ℃, and salt spray time stable at over 120 hours.

electro chemical passivation 

Electrochemical passivation (also known as anodic passivation) is a process in which a direct current electric field is applied between a metal and an inert electrode, and a passivation film is formed by controlling the current density or voltage to force an oxidation reaction on the metal surface. Its advantage lies in a more uniform film layer and controllable thickness, making it suitable for precision components with high corrosion resistance requirements, such as aerospace fasteners and medical devices.

The selection of chemical passivation or electrochemical passivation should be considered from the following three aspects: firstly, material characteristics - stainless steel and aluminum alloys require sufficient chemical passivation, while titanium alloys and high nickel alloys (such as Hastelloy) commonly use electrochemical passivation; The second is the scenario requirement - ordinary hardware components (such as screws and steel pipes) have low corrosion resistance requirements and low chemical passivation costs; Aerospace components (such as engine blades) require high corrosion resistance and reliability, and electrochemical passivation is necessary; Thirdly, there are environmental restrictions - chromate passivation is effective but causes significant pollution. In the context of the EU (such as RoHS directive) and stricter domestic environmental policies, chromium free passivation and electrochemical passivation (with recyclable solutions) are more favored.

Key control points of passivation process

The passivation process may seem simple, but it is actually a "delicate activity" - a temperature difference of 5 ℃ and a time difference of 1 minute can lead to significant differences in the performance of the film layer.

Taking nitric acid passivation of stainless steel as an example: when the temperature is controlled at 40-50 ℃, the proportion of Cr ₂ O3 in the film layer is the highest (>60%), and the corrosion resistance is the best; If the temperature is below 30 ℃, the nitric acid oxidation in the solution weakens, and chromium cannot be fully oxidized. The proportion of Fe ₂ O3 in the film layer increases (>50%), and the corrosion resistance decreases; When the temperature exceeds 60 ℃, the decomposition of nitric acid accelerates (producing NO ₂ yellow smoke), the solution concentration is unstable, and the film layer is prone to "flower spots" (local over corrosion).

Case: Due to a steam pipeline malfunction, the temperature of the passivation tank in a certain batch of workpieces suddenly rose to 70 ℃. As a result, a large number of white spots appeared on the surface of the workpieces. After less than 24 hours of salt spray testing, they rusted and several hundred pieces were directly scrapped, resulting in a loss of over 100000 yuan. Time "is another very important control point in passivation technology. The passivation time is too short, and the film layer is not completely formed; If the time is too long, the film layer may be "dissolved again". This' golden time 'needs to be determined comprehensively based on the material, solution concentration, and temperature. In fact, temperature and time need to be coordinated in the passivation process. Generally speaking, the higher the temperature, the faster the film formation rate and the shorter the required time.

For example, in the use of nitric acid passivation, it usually takes 10-30 minutes at high temperatures of 49-71 ℃; At room temperature of 21-38 ℃, it needs to be extended to 30-60 minutes. The concentration and pH value of the passivation solution are the core of the formula. The concentration and pH value of each component in the solution directly affect the "dynamics" and "direction" of film formation. Taking the commonly used nitric acid+hydrofluoric acid system for stainless steel passivation as an example: the concentration of nitric acid determines the "strength" of the oxidant, while the concentration of hydrofluoric acid determines the "cleaning" effect on free iron. When the concentration of nitric acid is 25% and the concentration of hydrofluoric acid is 3%, the pH value of the solution is about 1.5. At this point, the oxidation of chromium (producing Cr ₂ O3) and the dissolution of iron (removing free iron) reach equilibrium; If the concentration of hydrofluoric acid exceeds 5% and the acidity of the solution is too strong, it will preferentially corrode the stainless steel substrate (especially in areas with high carbon content), resulting in weak "rooting" of the film layer and a decrease in corrosion resistance. Before passivation, the surface of the workpiece needs to be cleaned, which is commonly referred to as acid washing. Because there are welding slag, splashes, oil stains, etc. left on the surface of the workpiece after welding or machining, if not cleaned thoroughly, it will affect the passivation effect.

Common process flows for acid pickling and passivation

Step 1: Surface purification pretreatment, using gasoline, alkaline solution or specialized cleaning agents to remove grease, using stainless steel specialized brushes to remove foreign objects, and rinsing with water with chloride ions ≤ 25mg/L.

Step 2: Acid washing, which can be soaked in acid solution. Common pickling parameters: nitric acid+hydrofluoric acid (such as HNO3 20%+HF 5%), treated at a temperature of 21-60 ℃; or treat with ammonium citrate at a temperature of 49-71 ℃ for 10-60 minutes. Alternatively, acid pickling paste can be applied evenly to the workpiece (with a thickness of about 2-3mm), and kept at room temperature for 40-60 minutes until the surface is uniformly silver white. Afterwards, rinse the surface of the workpiece with deionized water.

Step 3: Passivation, soaking in passivation solution can be used. Common passivation parameters: 20-50% nitric acid, 30-60 minutes for low temperature (21-38 ℃), 10-30 minutes for high temperature (49-71). Passivation paste can also be applied uniformly on the surface of the workpiece (with a thickness of about 2-3mm), and kept at room temperature for 1-3 hours. Afterwards, rinse the surface of the workpiece with deionized water.

Step 4: Rinse and dry. After pickling and passivation, the surface must be thoroughly rinsed with clean water (deionized water). You can use litmus paper to test and ensure that the pH value of the rinsing water is neutral (6.5-7.5) to be considered qualified. Finally, dry the surface of the workpiece with compressed air or wipe it dry with a clean cloth.

How to determine whether the passivation process is qualified? 

The surface of the workpiece should be uniformly silver white, without corrosion marks or uneven color. Blue dot test: This is the most rigorous on-site testing method. Drop the specialized reagent (potassium ferrocyanide+nitric acid) onto the surface, and if no blue dots appear within 30 seconds, the passivation film is qualified.

The difference between passivation and non passivation of stainless steel is best reflected in the salt spray resistance test. Stainless steel without passivation has poor salt spray resistance and visible red rust usually appears within 24-48 hours (depending on the testing conditions). After qualified passivation, the salt spray resistance of stainless steel is greatly improved, achieving a common and achievable industrial standard of no red rust for 3 days to several weeks (72 hours to over 200 hours).

Passivation is a key step in ensuring that stainless steel can unleash its corrosion resistance potential in salt spray environments.