In the fields of materials science, engineering, and manufacturing, the performance and longevity of metallic components are often threatened by corrosion-a natural process that gradually degrades metals through chemical or electrochemical reactions with the surrounding environment. To combat this issue, various surface treatment technologies have been developed, and passivation paste stands out as a versatile, efficient, and easy-to-apply solution. Unlike liquid passivants that may require complex dipping or spraying equipment, or solid passivation films that lack flexibility in application, passivation paste combines the advantages of targeted coverage, prolonged contact time, and adaptability to irregularly shaped surfaces. This article aims to provide a comprehensive overview of passivation paste, including its definition, composition, working mechanism, applications, selection criteria, application processes, quality control, and future trends. By exploring these aspects, readers will gain a clear understanding of what passivation paste is and why it has become an indispensable tool in modern industrial production.
Definition and core characteristics of passivation paste
To answer the question "What is passivation paste?", the first step is to clarify its basic definition and distinguish it from other passivation materials. Passivation paste is a semi-solid or paste-like functional material designed to form a thin, dense, and stable protective film (known as a passivation film) on the surface of metals. This film acts as a physical and chemical barrier, isolating the metal substrate from corrosive media (such as oxygen, water, acids, and salts) and thereby inhibiting or slowing down corrosion. Unlike liquid passivants (e.g., aqueous solutions of nitric acid or chromate) that flow easily and may be difficult to retain on vertical or curved surfaces, passivation paste has a higher viscosity. This viscosity allows it to adhere firmly to the metal surface for an extended period, ensuring sufficient time for the passivation reaction to occur.
Key components that define passivation paste
The unique properties of passivation paste are determined by its carefully formulated components, which work together to achieve effective passivation. These components typically include four core parts:
Passivator (active ingredient): This is the heart of passivation paste, responsible for initiating the passivation reaction with the metal surface. Common passivators vary depending on the type of metal to be treated. For stainless steel, nitric acid, citric acid, or their salts (e.g., sodium nitrate, ammonium citrate) are widely used; these substances react with the iron, chromium, and nickel in stainless steel to form a chromium-rich oxide film. For aluminum and its alloys, phosphoric acid, chromic acid, or zirconium-based compounds are preferred, as they generate a dense aluminum oxide or composite oxide film. For copper and copper alloys, benzotriazole (BTA) or its derivatives are commonly used, which form a chelating film with copper ions to prevent oxidation.
Binder (viscosity regulator): The binder is what gives passivation paste its paste-like consistency. It ensures that the paste adheres to the metal surface without dripping or flowing, even on inclined or vertical surfaces. Common binders include organic polymers (e.g., polyvinyl alcohol, acrylic resins) and inorganic binders (e.g., silica sol, alumina gel). Organic binders offer good flexibility and adhesion, while inorganic binders contribute to the high-temperature resistance of the passivation film.
Additives (performance enhancers): Additives are added in small quantities to optimize the performance of passivation paste. They include:
Corrosion inhibitors: Enhance the protective effect of the passivation film by suppressing local electrochemical reactions (e.g., sodium molybdate for stainless steel).
Thickeners: Adjust the viscosity of the paste to meet different application requirements (e.g., carboxymethyl cellulose).
Stabilizers: Prevent the passivator from decomposing or deteriorating during storage (e.g., urea for acid-based passivators).
Wetting agents: Improve the wettability of the paste on the metal surface, ensuring uniform coverage (e.g., non-ionic surfactants like Tween 80).
Solvent (carrier): The solvent dissolves or disperses the passivator, binder, and additives, forming a homogeneous paste. Water is the most common solvent due to its low cost, safety, and environmental friendliness. In some cases, organic solvents (e.g., ethanol, propylene glycol) are used for passivation paste intended for use in low-temperature environments or on water-sensitive metals.
Core characteristics that distinguish passivation paste
Compared with other passivation technologies, passivation paste has the following distinct characteristics:
High adhesion and targeted application: The paste's viscosity enables it to adhere to any metal surface-whether flat, curved, or irregular-without the need for specialized equipment. This makes it ideal for treating large components (e.g., industrial pipelines, ship hulls) or localized areas (e.g., weld seams, scratches) that are difficult to cover with liquid passivants.
Controllable reaction time: Unlike liquid passivants that may react too quickly (leading to incomplete film formation) or evaporate too soon, passivation paste remains in contact with the metal surface for an adjustable period (usually 30 minutes to 24 hours). This allows manufacturers to control the thickness and density of the passivation film according to specific requirements.
Low environmental impact (for modern formulations): Traditional passivation materials (e.g., chromate-based liquids) are highly toxic and pose risks to human health and the environment. However, most modern passivation paste formulations use non-toxic or low-toxic passivators (e.g., citric acid, zirconium compounds) and water-based solvents, reducing emissions of harmful substances and complying with global environmental regulations (e.g., the EU's REACH regulation).
Compatibility with post-treatment processes: After the passivation reaction is complete, passivation paste can be easily removed by washing with water or wiping, leaving a clean passivation film that does not interfere with subsequent processes such as painting, coating, or assembly.
Working mechanism of passivation paste
Understanding how passivation paste works is essential to grasping its value in corrosion protection. The core principle of passivation paste is to use its active components to trigger a controlled chemical reaction on the metal surface, resulting in the formation of a passivation film. This process can be divided into three sequential stages: surface activation, film formation, and film stabilization. Throughout these stages, passivation paste ensures that the reaction is thorough, uniform, and produces a high-quality protective film.
Stage 1: Surface activation by passivation paste
Before the passivation film can form, the metal surface must be free of contaminants (e.g., oil, rust, oxide scales) that could block the reaction between the passivator and the metal substrate. Passivation paste plays a dual role here: it not only acts as a passivator but also contains components that help activate the metal surface.
When passivation paste is applied to the metal surface, the acidic or chelating passivator in the paste first reacts with surface contaminants. For example, if the metal has rust (iron oxide), the nitric acid in a stainless steel passivation paste will dissolve the rust through an acid-base reaction: Fe₂O₃ + 6HNO₃ → 2Fe(NO₃)₃ + 3H₂O. At the same time, the passivator slightly etches the metal surface, removing a thin layer of the substrate (usually a few nanometers to micrometers thick). This etching process exposes a fresh, clean metal surface with high chemical reactivity, creating favorable conditions for the subsequent film formation reaction.
It is important to note that the surface activation by passivation paste is mild and controlled. Unlike strong acid pickling (which can over-etch the metal and cause pitting), the passivation paste's activator concentration and reaction time are carefully adjusted to avoid damaging the metal substrate while ensuring effective contaminant removal.
Stage 2: Film formation reaction driven by passivation paste
After surface activation, the passivator in passivation paste reacts with the metal ions (released from the fresh substrate) to form a passivation film. The type of reaction depends on the metal and the passivator used:
Oxidation-reduction reaction (for stainless steel and aluminum): For stainless steel, the chromium in the metal reacts with the nitric acid in the passivation paste to form chromium oxide (Cr₂O₃), which is the main component of the passivation film. The reaction equation is: 2Cr + 6HNO₃ → Cr₂O₃ + 6NO₂↑ + 3H₂O. This chromium oxide film is extremely dense (with a thickness of 5-20 nanometers) and has low electrical conductivity, preventing the metal from undergoing anodic dissolution (a key process in corrosion). For aluminum, the phosphoric acid in the passivation paste reacts with aluminum to form aluminum phosphate (AlPO₄) and aluminum oxide (Al₂O₃), which together form a composite film with excellent corrosion resistance.
Chelation reaction (for copper and copper alloys): For copper, the benzotriazole (BTA) in the passivation paste acts as a chelating agent, forming a stable five-membered ring complex with copper ions (Cu²⁺) on the surface. This complex film adheres tightly to the copper surface, blocking the contact between copper and oxygen or water, thereby preventing the formation of copper oxide (patina) and subsequent corrosion.
During the film formation stage, the viscosity of passivation paste plays a critical role. It ensures that the passivator remains in contact with the metal surface, preventing the reaction products from being washed away or evaporated prematurely. This prolonged contact allows the passivation film to grow gradually, becoming denser and more uniform.
Stage 3: Film stabilization and post-treatment of passivation paste
Once the passivation film is formed, the final stage involves stabilizing the film and removing any residual passivation paste. The passivation film, although initially formed, may contain small pores or incomplete areas. To address this, some passivation paste formulations include stabilizers that react with the film to fill these pores and enhance its stability. For example, in zirconium-based passivation paste for aluminum, the zirconium ions react with the aluminum oxide film to form a zirconium-aluminum composite oxide layer, which is more resistant to acid and alkali.
After the stabilization process, the residual passivation paste must be removed. This is typically done by washing the surface with water (for water-based paste) or wiping it with a solvent (for organic-based paste). The removal of residual paste is important because any remaining passivator (especially acidic or alkaline components) could cause localized corrosion over time. After cleaning, the metal surface is dried, leaving a thin, transparent passivation film that does not affect the appearance or dimensional accuracy of the component.
Application fields of passivation paste
Passivation paste's versatility-including its adaptability to different metals, ease of application, and excellent corrosion protection-has made it widely used in various industries. From aerospace to household appliances, passivation paste plays a critical role in extending the service life of metallic components and ensuring their reliable performance. Below are the key application fields, each highlighting how passivation paste addresses specific industry challenges.
Aerospace industry: Ensuring high reliability of components with passivation paste
The aerospace industry has extremely strict requirements for metallic components, as they must withstand harsh environments such as high altitude, temperature fluctuations, and exposure to hydraulic fluids and fuels. Common metals used in aerospace include titanium alloys, aluminum alloys, and stainless steel, all of which require effective corrosion protection. Passivation paste is particularly suitable for this industry due to its ability to treat complex-shaped components (e.g., engine parts, aircraft frames, and fasteners) that are difficult to process with liquid passivants.
For example, aircraft engine blades (made of titanium alloys) are prone to corrosion by salt spray (from oceanic environments) and high-temperature oxidation. Titanium-based passivation paste, containing oxalic acid or hydrogen peroxide as passivators, forms a dense titanium oxide film on the blade surface. This film not only resists salt spray corrosion but also maintains stability at high temperatures (up to 600°C), ensuring the blade's long-term operation. Additionally, passivation paste is used to treat weld seams on aircraft fuselages (made of aluminum alloys). Weld seams are often vulnerable to corrosion due to heat-affected zones and residual stresses; passivation paste is applied directly to these areas, forming a protective film that eliminates corrosion risks.
Automotive industry: Enhancing durability of parts with passivation paste
The automotive industry relies heavily on metallic components, from the chassis and engine to small parts such as bolts and nuts. These components are exposed to rain, road salt (in winter), and exhaust gases, making corrosion a major concern. Passivation paste is used in both the manufacturing of automotive parts and post-repair maintenance.
In the production of stainless steel exhaust systems, passivation paste is applied to the inner and outer surfaces of the exhaust pipes. The high-temperature-resistant passivation film (formed by chromic acid or nitric acid-based paste) prevents the exhaust system from corroding due to high-temperature exhaust gases and moisture. For aluminum alloy wheels, zirconium-based passivation paste is used before painting. The passivation film improves the adhesion of the paint to the wheel surface, preventing the paint from peeling off and exposing the aluminum to corrosion. Additionally, during automotive maintenance, passivation paste is used to treat rusted areas on the chassis. The paste dissolves light rust and forms a protective film, extending the chassis's service life.
Electronics and electrical industry: Protecting precision components with passivation paste
In the electronics and electrical industry, precision metallic components (e.g., printed circuit boards (PCBs), connectors, and transformer cores) are highly sensitive to corrosion. Even slight corrosion can cause poor electrical contact, short circuits, or equipment failure. Passivation paste is ideal for this industry because it can be applied in small, precise areas without damaging nearby electronic components.
For example, copper connectors on PCBs are prone to oxidation, which increases electrical resistance. Benzotriazole (BTA)-based passivation paste is applied to these connectors using a small brush or dispenser. The paste forms a thin chelating film on the copper surface, preventing oxidation and ensuring stable electrical conductivity. For transformer cores made of silicon steel sheets, passivation paste (containing phosphoric acid and silica sol) is used to form an insulating passivation film between the sheets. This film not only prevents corrosion but also reduces eddy current losses, improving the transformer's efficiency.
Household appliances and daily necessities: Improving user experience with passivation paste
Household appliances (e.g., refrigerators, washing machines, and cookware) and daily necessities (e.g., stainless steel water bottles, cutlery) are in constant contact with water, food, and cleaning agents, making corrosion and hygiene important issues. Passivation paste is used in the manufacturing of these products to ensure they are corrosion-resistant and safe for use.
For stainless steel cookware (e.g., pots and pans), citric acid-based passivation paste is applied to the inner surface. The paste forms a chromium-rich oxide film that is non-toxic and resistant to acid and alkali. This film prevents the cookware from reacting with food (e.g., acidic foods like tomatoes) and ensures that no harmful metals (e.g., nickel) leach into the food. For washing machine inner drums (made of stainless steel), passivation paste is used to treat the weld seams and inner surface. The passivation film resists corrosion by detergent and hard water, preventing the formation of rust spots and extending the washing machine's service life. Additionally, for stainless steel water bottles, passivation paste is applied to the inner wall to form a film that resists the growth of bacteria and prevents the bottle from developing a metallic taste.
Selection criteria for passivation paste
Not all passivation paste products are suitable for every application. The effectiveness of passivation paste depends on whether it is matched to the specific metal type, application environment, and performance requirements. Selecting the right passivation paste requires considering several key factors, which are detailed below.
Matching passivation paste to the target metal type
The most fundamental criterion for selecting passivation paste is matching it to the metal that needs to be treated. Different metals have different chemical properties, so they require passivators that can form stable films on their surfaces. Using the wrong passivation paste can result in ineffective film formation, or even damage to the metal.
Stainless steel: Stainless steel contains chromium (usually 10.5% or more), which is the key element for forming a passivation film. Passivation paste for stainless steel typically uses nitric acid, citric acid, or their salts as passivators. Nitric acid-based paste is suitable for high-grade stainless steel (e.g., 316L) and provides excellent corrosion resistance, but it is highly acidic and requires careful handling. Citric acid-based paste is non-toxic, environmentally friendly, and suitable for food-grade stainless steel (e.g., 304) used in cookware or food processing equipment.
Aluminum and aluminum alloys: Aluminum forms a natural oxide film, but this film is thin and porous, providing limited protection. Passivation paste for aluminum usually uses phosphoric acid, chromic acid, or zirconium compounds. Phosphoric acid-based paste is suitable for general-purpose aluminum components (e.g., window frames), while zirconium-based paste is preferred for high-performance applications (e.g., aerospace parts) due to its excellent high-temperature resistance.
Copper and copper alloys: Copper is prone to oxidation and tarnishing. Passivation paste for copper uses benzotriazole (BTA) or its derivatives as passivators. BTA-based paste forms a stable chelating film that is transparent and does not affect the appearance of copper, making it suitable for decorative copper products (e.g., jewelry, door handles) and electronic connectors.
Titanium and titanium alloys: Titanium needs passivation for harsh environments. Passivation paste for titanium uses oxalic acid (thick oxide film, acid/alkali resistance-suitable for chemical equipment/offshore drilling), hydrogen peroxide (non-toxic, residue-free-ideal for medical implants), or nitric acid (temperature-stable -50°C to 500°C-for aerospace engine parts).
Considering the application environment of passivation paste
Component environment dictates passivation film durability, so it's key for selecting passivation paste.
High-humidity/marine: Chloride ions cause corrosion. Stainless steel uses molybdate-modified nitric acid paste (molybdenum-chromium film repels chloride); aluminum uses zirconium paste (fills oxide pores to block chloride).
High-temperature: Over 300°C damages ordinary films. Stainless steel uses chromic acid paste (1200°C melting point film-for furnaces); titanium blades use oxalic acid paste (crystalline film resists thermal cracks).
Chemical corrosion: For sulfuric acid-stored stainless steel, use citric-nitric paste (softens oxide, forms dense chromium film); copper pipelines use BTA-silicone paste (hydrophobic layer protects from solvents).
Food/medical: Non-toxic, no residues. Food-grade stainless steel uses FDA-approved citric acid paste; medical titanium implants use hydrogen peroxide paste (biocompatible, bactericidal).
Aligning passivation paste with performance requirements
Component performance needs (thickness, appearance, conductivity) guide passivation paste selection.
Film thickness: Thin films (5-10 nm, low-concentration BTA paste-copper PCB connectors) balance conductivity; thick films (20-50 nm, high-concentration nitric acid paste-stainless steel offshore pipelines) boost corrosion resistance.
Appearance: Decorative parts need transparent films. Copper jewelry uses BTA-glycerin paste (retains luster); stainless steel sinks use citric acid paste (no yellowing).
Conductivity: Electrical parts need low-resistance films. Copper terminals use low-molecular-weight BTA paste; aluminum windings use phosphoric acid-carbon black paste (conductive network).
Paying attention to environmental and safety standards for passivation paste
Global regulations (REACH, EPA) demand eco-friendly, safe passivation paste.
Toxic limits: Modern paste avoids hexavalent chromium. Aluminum zirconium paste meets REACH SVHC standards (no chromate/heavy metals).
VOC emissions: Water-based paste (VOC <50 g/L, e.g., food-grade citric acid paste) replaces solvent-based to reduce air pollution.
Safety in use: Low-acid/neutral paste (pH 4-7, e.g., aluminum zirconium paste pH 5-6) is safer than strong acid (pH <1) which needs protective gear.
Application process of passivation paste
Correct application ensures good film quality; steps include pre-treatment, application, curing, post-treatment.
Pre-treatment: Preparing the metal surface for passivation paste
Remove contaminants for paste-substrate contact:
Degreasing: Alkaline solution (stainless steel) or ethanol (precision copper) removes oil.
Derusting: Weak acid (light rust) or mixed acid (heavy rust) pickles; control time (5-20 mins) to avoid over-etching.
Rinsing/drying: Deionized water rinse + hot air drying (50-80°C) prevents pre-corrosion/moisture interference.
Application: Applying passivation paste to the metal surface
Method depends on component shape/size:
Brushing: 0.5-2 mm layer for large parts/welds (covers heat-affected zones).
Spraying: 0.3-1 mm layer via low-pressure gun (complex shapes, multiple thin layers avoid cracking).
Dipping: 5-15 mins for small batches; drain excess paste to control thickness.
Curing: Controlling the reaction time and temperature of passivation paste
Curing conditions depend on paste/metal:
Room-temperature: Water-based paste (20-25°C, 30 mins-4 hours; stainless steel 1-2 hours, copper 30-60 mins) in well-ventilated area.
Heated: High-temperature paste (60-150°C, 15-60 mins; e.g., titanium blades at 120°C for 30 mins); gradual heating avoids thermal shock.
Post-treatment: Removing residual passivation paste and inspecting the film
Cleaning: Water-based paste (40-60°C deionized water rinse); organic-based (isopropanol wipe + water rinse); dry to avoid water spots.
Inspection: Visual (smooth, no cracks); adhesion (cross-cut test, no peeling); corrosion (24-72h salt spray, no rust); food-grade (2h 1% citric acid soak, no dissolution).
Quality control of passivation paste application
QC covers incoming inspection, in-process control, final inspection.
Incoming inspection: Verifying the quality of passivation paste
Formulation: Check active content (e.g., stainless steel nitric acid paste 15-25%, pH 0.5-1.5), viscosity (water-based 500-1500 cP).
Performance: Test sample (e.g., 304 steel + citric acid paste: 2h room temp cure + 24h salt spray, no rust); medical paste needs biocompatibility tests.
Storage: Check shelf life (6-12 months) and cool/dry conditions; discard expired/deteriorated paste.
In-process control: Monitoring key parameters of passivation paste application
Pre-treatment: Monitor degreasing/pickling time; maintain degreaser pH 10-12 (replace <9) and pickling pH 1-2 (replace >2).
Paste application: Track layer thickness (use a film thickness gauge) to ensure it stays within 0.3-2 mm (per method: brushing 0.5-2 mm, spraying 0.3-1 mm). For spraying, check gun pressure (0.2-0.3 MPa) to avoid uneven coverage; for dipping, control immersion time (5-15 mins) to prevent excess paste buildup.
Curing process: For room-temperature curing, record ambient temperature (20-25°C) and humidity (<60%)-high humidity slows reaction. For heated curing, use a temperature controller to keep oven temp within ±5°C of the target (60-150°C) and monitor heating rate (5-10°C/min) to avoid thermal shock. Log curing time (15 mins-4 hours) to ensure full film formation.
Final inspection: Evaluating passivated components
After post-treatment, comprehensive inspection ensures the passivation film meets standards:
Film quality check: 除 visual and adhesion tests (cross-cut, no peeling), use a nanometer thickness gauge to verify film thickness (5-50 nm, per requirement). For high-precision parts (e.g., electronics), use a scanning electron microscope (SEM) to check for micro-cracks or pores.
Corrosion resistance validation: Conduct salt spray tests (24-72h in 5% NaCl solution) for general components; for chemical-resistant parts, add targeted tests (e.g., 24h soak in 10% sulfuric acid for stainless steel tanks). No rust, pitting, or film dissolution is allowed.
Residue detection: Use Fourier-transform infrared spectroscopy (FTIR) or ion chromatography to check for toxic residues (e.g., heavy metals <10 ppm, residual passivator <50 ppm). For food/medical parts, pass a total organic carbon (TOC) test to ensure no harmful organics remain.
Functional verification: For electrical components (e.g., connectors), measure conductivity (resistivity <10⁻⁶ Ω·cm) with a multimeter; for high-temperature parts (e.g., engine blades), conduct a thermal cycle test (-50°C to 500°C, 10 cycles) to confirm film stability.
Future trends of passivation paste
As industrial demands for efficiency, eco-friendliness, and smart performance grow, passivation paste is evolving in three key directions:
Eco-friendly and low-carbon formulations: Development of bio-based binders (e.g., starch-derived thickeners) and solvent-free pastes to eliminate VOC emissions entirely. Research on "green passivators" (e.g., plant extracts like tea polyphenols for copper) to replace synthetic acids, reducing environmental impact.
High-efficiency and multi-functional paste: Integration of nanomaterials (e.g., TiO₂ nanoparticles) to enhance film density-cutting curing time by 50% (from 2h to 1h for stainless steel). Addition of self-healing microcapsules (filled with passivator) that rupture when the film cracks, repairing defects automatically and extending component lifespan.
Smart and digital compatibility: Embedding temperature/humidity sensors in paste for real-time monitoring of curing conditions, linked to industrial IoT (IIoT) systems to adjust parameters remotely. Development of "color-changing paste" that shifts hue when film thickness is insufficient, enabling visual quality control without tools.
Core Value and Sustainable Role of Passivation Paste
Passivation paste is a critical corrosion protection solution, with its value rooted in targeted selection (matching metal, environment, performance), standardized application (pre-treatment to post-treatment), and strict quality control. As it evolves toward eco-friendliness and smart functionality, it will continue to support industries like aerospace, electronics, and medical devices-ensuring metallic components perform reliably in harsh conditions while meeting global sustainability standards.