Técnicas de passivação de aço inoxidável explicadas

Stainless Steel Passivation Techniques Explained

April 27, 2024 Roberto Magalhães

1. Overview

Passivation of stainless steel involves applying a film of chromium oxide to the surface and welds of the steel plate, thereby increasing the corrosion resistance of stainless steel.

Passivation also serves as an effective decontamination method. For example, iron particles deposited on the metal surface and in welds can be removed in this way (these iron particles are often formed due to cutting, shaping, equipment friction or the action of metal brushes).

Acid washing before passivation can eliminate all contaminants and selectively remove smaller corrosion-prone areas on the metal surface. For effective acid washing and passivation, all organic contaminants and grease must be removed beforehand.

Stainless Steel Passivation Techniques Explained

Therefore, the commonly used procedure is as follows:

A. Pre-cleaning/degreasing
B. Primary Cleaning
C. Acid washing
D. Secondary Cleaning
E. Passivation/Decontamination
F. Final Cleaning and Drying

Under normal circumstances, products should be coated using a spraying process as much as possible. For smaller products or pipe fittings, bath immersion treatment should be considered first.

In small areas (welding, repairs), or when spray coating is harmful to certain products (such as the surface of pre-installed tubular heat exchangers), it is preferable to use a paste.

The production workshop must prepare a written document based on this manual and submit it to the buyer for confirmation.

Observation:

Passivating agents should not contain hydrochloric acid or chlorides. Acid washing and passivation may be ineffective at low temperatures, therefore these processes must be carried out at sufficiently high ambient temperatures (>10°C).

In all circumstances, treatments must be carried out under the guidance of the supplier. The water used in the process (such as bath, diluent, cleaning liquid, etc.) must be treated to ensure a low chloride content (the theoretical maximum chloride content is 30ppm).

2. Pre-Cleaning and Degreasing

To ensure effective pickling and passivation, it is imperative to remove all organic contaminants from the metal surface, such as grease and other debris. Organic pollutants can hamper the pickling and passivation process and pose a potential pitting corrosion risk.

A pre-cleaning agent is sprayed onto the metal surface for cleaning and degreasing purposes. Then, it is essential to rinse with a high-pressure water gun to improve the quality of subsequent treatments.

The effectiveness of pre-cleaning can be checked using the water film method.

3. Acid pickling and passivation in bath solution

The following procedures are carried out after pre-cleaning and degreasing:

3.1 Each product is immersed in the following solution:

Nitric Acid 36℃ Be 100 liters
65% hydrofluoric acid or 20 liters
Sodium fluoride 20kg
Water 900 liters

3.2 If the treatment solution is at 60°C, a ten-minute soak is sufficient, while at room temperature a two-hour soak is required.

3.3 After immersion, the product must be quickly rinsed with water until the pH of the effluent equals the pH of the rinse water.

3.4 Each product must be reimmersed in the following solution:

Nitric Acid 36℃ Ser 250 liters
Water 750 liters

3.5 The product immersion time is as follows:

15 minutes when the solution temperature is 50°C
2 hours at room temperature

3.6 After immersion, the product must be quickly rinsed with water until the pH of the effluent equals the pH of the rinse water.

4. Application of Pickling Passivation Paste

Certain small areas, such as weld seams and heat-sensitive spots, should be treated with pickling passivation paste. This method is also worth considering when dipping or spray coating is inconvenient.

Pickling passivation paste is particularly suitable for spot treatment after repairs or for maintenance of equipment parts.

4.1 Pickling with Pickling Paste

The pickling paste used for stainless steel is a mixture of nitric acid and hydrofluoric acid with an adhesive.

Use an acid-resistant brush to apply the paste to the weld seam and smooth it with a stainless steel brush. Rinse with a high-pressure water gun before the paste dries.

4.2 Passivation with Passivation Paste

The passivation paste used for stainless steel is a mixture of nitric acid and hydrofluoric acid with an adhesive.

Use an acid-resistant brush to evenly apply the passivation paste to the pickled areas.

Wait 3-4 hours after applying this paste and brush lightly with a nylon brush. Rinse with a high-pressure water gun before the paste dries, then dry the metal surface.

5. Spray coating treatment

The pickling liquid and colloid used for spray coating mainly consist of nitric acid (20-25%) and hydrofluoric acid (about 5%) with adhesive and surfactant, forming a solution with suitable concentration and rheological properties.

The composition of the passivation liquid and colloid used for spray coating is similar to that of pickling spray coating liquid, but does not contain hydrofluoric acid.

The specific steps are as follows:

5.1 Pickling

After careful pre-cleaning and degreasing (see Section 2 for details), use acid-resistant equipment to spray an even layer of stripping agent onto the dry metal surface. Allow the pickling agent to work for a period under the supplier's direction.

If necessary, carefully scrub dark welds and heat-sensitive areas with a stainless steel brush, removing any stains. Use a high-pressure water gun to rinse well and check the metal surface for residue.

5.2 Passivation

After oxidation and immediate cleaning, an even layer of passivating agent should be spray applied to the dry metal surface using acid-resistant equipment. Under the supplier's guidance, the acid wash should react for a period of time.

If necessary, on heavily colored weld seams and heat sensitive areas, a stainless steel brush should be used to scrub meticulously to remove any discolorations.

The metal surface must then be thoroughly rinsed with a high-pressure water gun, ensuring that no residue remains, and completely dried.

This process follows the technical specifications for acid washing and passivation of stainless steel.

6. Acid Washing and Passivation Products and Suppliers

(omitted)

7. Metal surface treatment

The following equipment or components must undergo acid washing and passivation treatment:

From the chassis, walls and evaporator housing to the upper defroster;
The inner walls of seawater and brine pipes;
All weld seams and heat sensitive areas;
If the weld seam is damaged and not yet painted, the outer surface of the seam does not need to be treated;
The inner surface of the condenser;
The internal surfaces of the degassing tower, chemical tanks and seawater filters.

Before treatment, the heat exchanger tubes must be carefully protected to prevent erosion by acid washing and passivation agent.

Cleaning, acid washing and passivation must be carried out in the production workshop before equipment is shipped.

Even so, throughout the useful life of the equipment, from initial use to full operation, it will be necessary to regularly inspect the treated surfaces. If discoloration or corrosion appears, re-treatment is necessary.

8. Treatment results and quality control

Upon completion of the pickling passivation process in the workshop or construction site, the contractor must submit separate reports on the pre-cleaning, pickling, passivation and final treatment results.

Each step of the operation, such as the treatment agents used, including diluents, along with the treatment methods and duration, must be recorded.

It is best to carry out the final inspection within two days after the pickling passivation is completed.

8.1 Visual Inspection

After cleaning and degreasing, the metal surface must be free of any grease or organic matter.

This can be detected by the water film method: a thin film of water forms on the surface of the metal and, if contamination exists, the water film will break in the contaminated area.

All passivated metal surfaces must be the color of the pure metal.

For more effective inspection, high-power lighting can be installed at the inspection location.

8.2 Iron Particle Detection

This detection method is extremely sensitive. Even small amounts of iron particles on a completely clean metal surface can be detected.

Both colored areas and suspicious areas should be tested using this method.

Attention: This method is prohibited on the surface of evaporator distillate products.

Iron particle detection method:

This method is described in ASTM A380.

The test liquid is prepared by mixing the following components:

Distilled water 94% 1000cm ³
60-67% nitric acid 3% 20cm ³
Potassium ferricyanide 3% 30g

Clean the metal surface with a clean cloth and deionized water.

Use a non-stainless steel sprayer to spray the prepared solution.

A few minutes later, if it appears blue-green, it indicates contamination, while yellow indicates no contamination.

Record the test situation, immediately wash with deionized water and wipe with a cloth. All test substances must be removed.

All contaminated areas must be repassivated.

If the area of contamination (blue-green) is very large, a more complete test should be carried out under the direction of the purchaser.

9. Preventive Measures

All products are acidic and dangerous, therefore all operators must wear masks, safety glasses, rubber gloves, aprons and boots.

Adequate ventilation must be maintained at all times. In any circumstance, the supplier's guidelines must be followed.

Potassium ferrocyanide is a simple, non-toxic cyanide. However, when heated or in contact with concentrated acid, it releases toxic cyanide gas.

If the air is contaminated with dust, carbon steel or grease and organic particles for a long time, cleaning, pickling and passivation will be ineffective. The entire process must maintain a high level of cleanliness.

If necessary, equipment can be appropriately treated with stainless steel pickling and passivation techniques to prevent exposure to contaminated air.