Does the sequence of passivation treatment and degreasing treatment have any impact on the degreased

The sequence of passivation treatment and degreasing treatment has a significant impact on the performance of degreased seamless pipes. This needs to be determined based on the surface condition and process requirements. The core logic is as follows:

1. Conventional process sequence and its impact

1.1 First degreasing then passivation (recommended process)

Advantages: Degreasing (using alkaline cleaners or solvent cleaners) removes organic substances such as oil and cutting fluids first, avoiding contamination of the passivation solution by grease (for example, if the oil content in the nitric acid passivation solution is > 0.5%, the film formation will be uneven).

Case: For 304 stainless steel pipes, after being treated with an alkaline degreasing agent (pH 10-12, temperature 50-60°C), the uniformity of the passivation film thickness improved by 30%, and the salt spray test corrosion resistance time extended to over 800 hours. 

2. Pretreatment followed by degreasing (prohibited scenario)

Risk: The passivation film (such as chromium oxide film with a thickness of 2-5nm) is prone to be dissolved by the strong alkaline components in the degreasing agent (NaOH concentration > 5%), resulting in a damage rate of the film layer exceeding 50% and a decrease in corrosion resistance. 

II. Special Process Adaptation Scenarios

- When there is stubborn oxide scale on the surface:

Sandblasting / Shot peening to remove the oxide scale → 2. Degreasing (removing the dust residue from sandblasting) → 3. Passivation (forming a complete protective film).

Note: If passivation is done first, the oxide scale will hinder the adhesion of the film layer, resulting in an increased local incomplete passivation rate. 

III. Remedial Measures for Sequence Errors

If the pretreatment is carried out by passivating first and then degreasing, immediately rinse with deionized water (conductivity <10 μS/cm), then lightly activate the surface with 5% nitric acid solution (temperature 40℃, time 5 minutes), and then re-passivate. 

Key parameter comparison

De-greasing followed by passivation

Passivation film integrity: ≥ 95%

Corrosion resistance (NSS test): ≥ 1000 hours without rust

Surface adhesion (scratch test): 0 grade (ISO 2409) 

First passivation, then degreasing

Complete degree of passivation film: <50%

Corrosion resistance (NSS test): <200 hours of rusting

Surface adhesion (scratch test): 3-4 grades (film layer peels off) 

Conclusion: Degreased seamless pipes must follow the sequence of "degreasing first and then passivation" to ensure that the passivation film directly adheres to the clean metal surface, maximizing the corrosion resistance. If surface mechanical treatment (such as shot blasting) is involved in the process, it must be completed before degreasing to avoid secondary contamination.