Overview
ISO 8504-4:2022 - "Preparation of steel substrates before application of paints and related products - Surface preparation methods - Part 4: Acid pickling" describes the typical acid pickling method used in shop facilities to prepare steel substrates prior to painting. The standard is generally applicable to new steelwork and focuses on removing rust and mill scale from steel surfaces (including internal, external and complex geometries) while being suitable for steel that would deform under abrasive blasting. It also covers general process constraints (limited removal of heavy oils), health and safety considerations, and quality checks.
Key topics and technical requirements
- Scope and purpose: Acid pickling as a surface preparation method for steel substrates to improve coating performance by removing rust and mill scale.
- Pickling liquid composition: Guidance on etching components (inorganic acids such as hydrochloric, sulfuric, phosphoric and organic acids like citric or oxalic), inhibitors to control metal pickling rate, and surfactants to aid degreasing. Examples and formulations are provided in Annex A (examples and process conditions).
- Equipment and bath requirements: Tanks, pipes and other bath-contact materials should be corrosion-resistant and suitable for acidic environments.
- Bath analysis and control: Regular monitoring of pickling liquid concentration and timely replenishment to maintain process effectiveness.
- Process steps: Preparation (removal of heavy oils), degreasing (typically alkaline with surfactants), fresh water rinsing/washing, immersion pickling (placement, time, temperature), post-treatment (neutralization, conversion layer or phosphate layer improvement), drying, and final surface assessment.
- Quality assurance: Assessment methods for cleaned surfaces are referenced; consistent documentation and procurement specifications are recommended.
- Health, safety and environmental: Emphasis on trained personnel, compliance with national hazardous-materials and waste-management regulations, and use of appropriate PPE and controls.
Applications and users
- Practical for fabricators, coating specifiers, corrosion engineers, surface preparation contractors, inspection and QA teams, and paint applicators.
- Used when abrasive blasting is unsuitable (thin or deformable steel), for complex internal surfaces, or where decorative/aesthetic pickling is desired.
- Typical industries: heavy fabrication, structural steel, manufacturing of complex assemblies, and maintenance shops.
Related standards
- ISO 8504 series (other surface preparation methods)
- ISO 8502 series (tests for assessment of surface cleanliness)
- ISO 8501 and ISO 8503 series (surface cleanliness and profile assessment)
Keywords: ISO 8504-4:2022, acid pickling, surface preparation, steel substrates, pickling liquid, inhibitor, degreasing, bath analysis, coating preparation.
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