Bionil P1, P2 and P3 are three different pickling products that remove the post-weld oxide layer, discoloration and oxidation residues from stainless steel surfaces. All three share the same goal: preparing the surface for the passivation step. The difference between them is which formula delivers the most efficient result in which application scenario.
In this guide, we cover the formulation and application differences of the three products, which one should be preferred in which scenario, and a decision matrix based on field practice for the right product selection. The comparison table below helps you make the decision in 30 seconds.
Key Takeaways
- Bionil P1 is designed for immersion, P2 as a localized weld seam paste, and P3 as a large-surface spray
- The three main determinants for choosing the right product: surface geometry, type of contamination and application speed
- All three products are compatible with AISI 304, 316, 316L and 321 alloys; only the reaction time changes with the alloy
- After pickling, the surface must always be rinsed with pressurized clean water, followed by passivation with Bionil PR
- PPE (gloves, goggles, face shield, apron) and adequate ventilation are mandatory in all pickling operations that use acid
What do the three products do?
The term pickling refers to the removal of the oxide and chromium-depleted zone formed on the stainless steel surface using an acid-based solution. The yellowish, brown or blue discoloration (heat tint) visible on the surface after welding, together with the chromium-depleted layer beneath it, constitutes corrosion initiation points. Pickling dissolves this layer in a controlled manner and exposes the healthy stainless surface underneath.
The pickling trio of the Bionil family performs this task in different forms:
- Bionil P1 Pickling Paste: Free-flowing liquid; for small parts and batch production in an immersion tank
- Bionil P2 Pickling Paste: Viscous paste; for vertical surfaces and localized weld seams
- Bionil P3 Pickling Spray: Spray form; for large surfaces and fast application
Formula differences
Although the active ingredient group of the three products is similar (acid mixture + surface-active additives), they differ in viscosity, acid concentration, reaction speed and application method.
- P1: Low viscosity, free-flowing; it is meant to fully envelop the part inside the bath. The dilution ratio is adjustable (typically up to 1:1 with water depending on the degree of contamination).
- P2: High viscosity, gel-like consistency; it does not run on vertical surfaces and is applied with a brush or spatula. It remains in contact with the surface for an extended time, concentrating on the localized spot.
- P3: Spray-compatible viscosity suited for uniform spraying; covers a large area in a short time. It typically provides effective cleaning with a contact time of 20-40 minutes on the surface.
Comparison table
| Property | Bionil P1 | Bionil P2 | Bionil P3 |
|---|---|---|---|
| Form | Free-flowing liquid | Paste / gel | Spray |
| Application method | Immersion | Brush / spatula | Spray applicator |
| Surface type | Small parts | Localized weld seam / vertical | Large horizontal / vertical |
| Reaction (AISI 304) | 30-60 min | 15-45 min | 20-40 min |
| Reaction (AISI 316/321) | 60-90 min | 30-90 min | 40-80 min |
| Dilution | Up to 1:1 with water | Not diluted | Not diluted |
| Post-treatment rinse | Pressurized water | Pressurized water | Pressurized water |
| Passivation | Bionil PR recommended | Bionil PR recommended | Bionil PR recommended |
Reaction times vary depending on the degree of surface contamination, ambient temperature and the chromium-molybdenum content of the alloy. Calibrating the time on a small test area of a new part is standard industry practice.
When to use Bionil P1 (immersion)?
Bionil P1 is a free-flowing pickling solution applied in an immersion tank. The scenarios where it is most efficient:
- Small and medium-sized parts: Flanges, elbows, small fasteners, valve bodies, tee fittings
- Batch production: High-volume manufacturing of the same part; parallel processing in a single bath
- Complex geometry: Parts whose internal bends, holes and cavities cannot be reached with a manual brush
- Heavy oxidation: Surfaces with long-term storage exposure, incipient corrosion or intense weld heat tint
During application, parts are placed into a bath prepared in acid-resistant plastic containers; the bath is agitated from time to time, and once the reaction time is complete, the parts are rinsed with high-pressure clean water.
For product details: Bionil P1 Pickling Paste page.
When to use Bionil P2 (paste)?
Bionil P2 Pickling Paste stands out in vertical and localized applications thanks to its viscous formulation:
- Localized weld seams: Cleaning only the affected area of weld seams on tanks, boilers or pipes
- Vertical and overhead surfaces: Applicable at any angle thanks to its non-running consistency that stays on the surface
- T-joints, corners and narrow areas: Localized spot application with a brush or spatula
- Aesthetics-driven applications: Manufacturers who want to confine the pickling mark to the weld seam area only
The paste structure of P2 ensures that the product contacts only the area that needs cleaning; there is no unnecessary acid contact with the surrounding healthy surface. This saves material and reduces the amount of waste.
For product details: Bionil P2 Pickling Paste page.
When to use Bionil P3 (spray)?
Bionil P3 Pickling Spray is optimized for large-surface and fast-application scenarios:
- Large tank interiors: Situations requiring uniform 360° coverage inside a tank
- Wide stainless steel sheets: Large flat surfaces such as panels, doors and cabinets
- Mixed horizontal and vertical surfaces: Treating both floor and wall surfaces with a single applicator
- High-throughput requirements: Production lines that need to clean more m² per unit of time
In P3 application, the spray applicator is held 20-30 cm from the surface and the product is sprayed so as to form a uniform film. Once the reaction is complete, the surface is rinsed with pressurized clean water.
For product details: Bionil P3 Pickling Spray page.
Scenario-based selection guide
The examples below show product matching for typical production scenarios in the industry. Times and technical details may vary depending on the surface and the alloy.
| Production scenario | Recommended product | Explanation |
|---|---|---|
| Stainless steel tank manufacturer (large surface) | Bionil P3 + Bionil PR | Spray for the large interior surface, followed by passivation |
| Localized tank weld seam | Bionil P2 + Bionil PR | Only the seam area is cleaned, then passivation is applied |
| Flange / valve / small-part manufacturer | Bionil P1 + Bionil PR | Batch processing in an immersion bath |
| Industrial kitchen manufacturing | Bionil P2 + P3 (depending on the area) | P2 for seams, P3 for large panels |
| Pharmaceutical / food CIP tank interior | Bionil P3 + Bionil PR (citric acid passivation) | Uniform coverage by spray, followed by low-emission passivation |
| Automotive welded spare parts | Bionil P1 (batch) or P2 (localized) | Selection based on production volume |
| On-site maintenance / repair weld seams | Bionil P2 + Bionil PR | Mobile application, the paste holds on vertical surfaces |
When deciding, ask yourself these three questions:
- Is the surface geometry large or localized?
- Can the parts be placed into a tank/bath?
- Is your production volume measured in units or in square meters?
Post-pickling rinsing and passivation
After pickling with P1, P2 or P3 is complete, the surface must always be checked for acid residues and rinsed with pressurized clean water. The Bionil Ferronil test can then be applied to check for free iron. As the final step, passivation with Bionil PR restores the corrosion resistance of the stainless surface to its original value.
For the detailed passivation workflow, see our passivation guide. For how pickling products behave on different alloys, the AISI 304 vs 316 guide will also be helpful.
Safety and PPE
Since all three products contain acid, the following equipment and precautions are mandatory during application:
- Acid/alkali-resistant chemical gloves (nitrile or neoprene)
- Chemical goggles with side protection + face shield
- Acid-resistant apron or disposable coverall
- Appropriate respiratory protection (FFP3 or organic gas filter if required)
- Good ventilation; a local exhaust ventilation (LEV) system if possible
- Access to an emergency eye wash and body shower
The manufacturer's safety data sheets (SDS/MSDS) must be read by all operators before application. You can access Bionil MSDS and TDS documents here.
Frequently asked questions
Can P1, P2 and P3 be used interchangeably? Although they belong to the same chemical family, their formulations are optimized for different scenarios, so they are not full substitutes; the choice should be made based on the application type.
Can P3 be sprayed over a seam already treated with P2? In general it is not necessary; in most application plans, a single product is selected.
What if the reaction time was not sufficient? If discoloration or staining is still visible on the surface, the product should be renewed and reapplied; excessively extending the contact time or leaving the same chemical on until it dries is not recommended.
How is pickling product waste disposed of? It must be neutralized in accordance with local waste regulations and sent via a licensed waste carrier; it must not be discharged directly into the sewer.
Conclusion
The Bionil P1, P2 and P3 family covers the full range of stainless steel pickling needs in three different forms, according to surface geometry, production volume and application method. The right product choice directly affects cleaning quality, production speed, chemical consumption and waste load.
Get technical consultancy from the Bionil technical team to determine the most suitable pickling strategy and product combination for your production line. Including product trials, training and process optimization support, we build a plan tailored to your needs.
This article was prepared by Bionil Technical Team. Last updated: May 25, 2026.
