Abrasive blast cleaning prior to hot-dip galvanizing serves contradictory purposes depending on steel chemistry: increasing coating thickness on low-reactivity steels while decreasing thickness on highly reactive compositions. This paradox creates specification challenges for engineers seeking to optimize coating performance and control costs. To quantify these effects and provide evidence-based guidance, the American Galvanizers Association initiated a comprehensive research program examining how pre-galvanizing blast cleaning influences coating development across the steel reactivity spectrum.
The Steel Chemistry Paradox
Hot-dip galvanized coating thickness results from zinc-iron metallurgical reactions occurring during steel immersion in molten zinc at approximately 840°F (449°C). Steel composition—particularly silicon, phosphorus, and aluminum content—dramatically influences reaction kinetics and final coating characteristics.
Strategic applications of pre-galvanizing blast cleaning:
- Coating thickness enhancement for low-silicon or aluminum-killed steels that naturally produce thin coatings below specification minimums
- Coating thickness limitation for highly reactive steels prone to excessive thickness and brittle alloy layer formation
- Surface profile optimization for improved coating adhesion and uniformity
The identical surface preparation technique produces opposite outcomes depending on base metal chemistry, necessitating steel-specific evaluation rather than universal specification practices.
Research Objectives and Methodology
In September 2021, the AGA launched a systematic investigation to resolve longstanding questions about abrasive blasting effectiveness across steel grade categories. The study aimed to:
- Compile existing research: Comprehensive literature review documenting historical investigations and field observations
- Quantify coating thickness variations: Measure actual thickness differences between blasted and unblasted surfaces on identical steel samples
- Establish cost-benefit parameters: Define application scenarios where pre-galvanizing blast cleaning justifies additional processing expense
- Predict brittle coating mitigation: Determine blast cleaning effectiveness for preventing thick, brittle coatings (typically >8-10 mils) prone to flaking or spalling
Test Sample Selection and Steel Chemistries
Research samples represented steel grades commonly encountered in markets where coating thickness control presents practical challenges:
Material categories tested:
- Low-silicon structural steel (Si <0.04%): Prone to thin coatings below ASTM A123 minimums
- Sandelin-range steel (Si 0.04-0.15%): Unpredictable reactivity with high variability
- Slightly reactive steel (Si 0.22-0.25%): Moderate thickness elevation above baseline
- Highly reactive steel (Si >0.25%): Excessive coating development with brittle characteristics
- Recycled steel compositions: Variable chemistry reflecting scrap-based production
- Structural shapes and signposts: Geometry-specific behavior assessment
Each test specimen was divided into blasted and unblasted zones, enabling direct comparison on identical base metal. This experimental design eliminated steel chemistry variability and isolated blast cleaning effects.
Surface Preparation Parameters
Abrasive blast cleaning prior to galvanizing does not require specification of surface cleanliness degree or precise profile parameters as required for organic coating systems. However, consistent methodology ensures reproducible results:
Blast cleaning specification:
- Abrasive media: Steel grit G25
- Cleanliness standard: Commercial Blast Cleaning per SSPC-SP 6/NACE No. 3 (used as reference guideline)
- Profile depth achieved:
- Plate and beam samples: 3.0 mils
- Signpost structures: 2.0 mils
This profile range represents practical field conditions achievable with conventional blasting equipment without specialized controls. Lower profiles on tubular signposts reflect geometric access limitations typical of hollow structural sections.
Galvanizing Process Variables
To assess immersion time influence—a critical factor in reactive steel coating development—duplicate sample sets underwent different galvanizing schedules:
Batch 1: Normal immersion time Standard dip duration based on steel section thickness and thermal mass, representing typical production galvanizing
Batch 2: Extended immersion time Double the normal immersion duration, simulating scenarios such as:
- Heavy structural members requiring extended heat-through time
- Complex assemblies with thick and thin sections galvanized together
- Hollow components with inadequate venting causing prolonged submersion
- Reactive steel requiring extended dip time to ensure complete coating coverage
This dual-timeline approach revealed whether blast cleaning effects remain consistent or amplify with increased zinc-steel contact time.
Quantified Research Results
Testing demonstrated that abrasive blasting impact varies systematically with steel silicon content and immersion duration. Table 1 summarizes coating thickness response across the reactivity spectrum:
Low-Reactivity Steels (Silicon <0.04%)
Effect of blast cleaning: Significant coating thickness increase
Low-silicon and aluminum-killed steels produce thin, well-adherent coatings through a zinc-iron diffusion mechanism rather than reactive alloy layer growth. Surface roughening from blast cleaning increases effective surface area and promotes mechanical interlocking, substantially enhancing coating development.
Key findings:
- Coating thickness increases of 25-60% achieved on blasted surfaces
- Effect remains consistent regardless of immersion time
- Blast cleaning reliably elevates thin coatings above ASTM A123 minimums
- Enhanced coating adhesion from mechanical keying into surface profile
Practical applications:
- Structural steel with silicon content <0.03% consistently producing thin coatings
- Aluminum-killed steel grades (ASTM A572, certain A36 heats)
- Projects specifying coating thickness above standard minimums for extended service life
- Applications where thin coatings risk non-compliance with specification requirements
Sandelin-Range Steels (Silicon 0.04-0.15%)
Effect of blast cleaning: Variable increase depending on immersion time
The Sandelin range represents a transition zone between controlled diffusion coating and reactive alloy layer formation. Steel within this composition range exhibits unpredictable behavior, with coating thickness varying substantially between production heats.
Key findings:
- Short immersion time: Negligible to minor coating thickness increase (5-15%)
- Extended immersion time: Moderate coating thickness increase (20-35%)
- High variability within the Sandelin range makes prediction difficult
- Blast cleaning effects less pronounced than with low-silicon steels
Practical considerations:
- Sandelin-range steel should be avoided when possible through steel specification
- If unavoidable, blast cleaning provides modest thickness enhancement
- Extended kettle time amplifies blast cleaning effects
- Coating uniformity improvements may justify blasting even with minimal thickness change
Slightly Reactive Steels (Silicon 0.22-0.25%)
Effect of blast cleaning: Negligible effect on coating thickness
Moderate silicon content (0.15-0.28%) promotes controlled reactive coating formation without excessive thickness. This represents optimal steel chemistry for hot-dip galvanizing, producing durable coatings with predictable characteristics.
Key findings:
- Minimal coating thickness reduction (0-10%) observed
- Some test samples showed slight thickness increase rather than decrease
- Immersion time variation produced minimal additional effect
- Blast cleaning does not provide significant thickness control benefit
Specification implications:
- Pre-galvanizing blast cleaning generally not cost-effective for thickness control
- Consider blast cleaning only for aesthetic surface uniformity requirements
- Proper steel specification provides better thickness control than surface preparation
- Focus quality control efforts on steel chemistry verification rather than blast cleaning
Highly Reactive Steels (Silicon >0.25% or Recycled Steel)
Effect of blast cleaning: Coating thickness reduction, amplified by extended immersion
High silicon content drives excessive zinc-iron alloy layer formation, producing thick, brittle coatings prone to cracking or spalling during cooling, fabrication handling, or field service. These coatings often exceed 8-10 mils where brittleness becomes problematic.
Key findings:
- Short immersion time: Minor thickness reduction (10-20%)
- Extended immersion time: Significant thickness reduction (35-50% achieved on some samples)
- Blast cleaning effectiveness increases with longer kettle exposure
- Further investigation needed to establish upper limits of silicon content where blast cleaning remains effective
Critical applications:
- Large structural assemblies requiring extended galvanizing time
- Hollow sections with restricted venting prolonging immersion
- Reactive steel grades unavoidable due to material availability
- Projects with history of coating flaking or spalling issues
Important caveat: Research continues to establish silicon content thresholds where blast cleaning loses effectiveness. Extremely reactive steels (Si >0.35%) may exceed blast cleaning's mitigating capability.
Understanding the Mechanism
The contradictory effects of blast cleaning across the reactivity spectrum result from different coating formation mechanisms:
Low-silicon steel coating formation: Zinc penetrates roughened surface valleys through diffusion, increasing coating mass and mechanical adhesion. Surface area increase from profile roughness provides additional sites for zinc-iron interdiffusion.
Highly reactive steel coating formation: Silicon catalyzes rapid iron dissolution into molten zinc, forming thick zeta (ζ) alloy layers. Blast-induced surface work hardening and residual compressive stress in the surface layer may slow iron dissolution rates, moderating reaction kinetics and limiting excessive alloy layer growth. The mechanism requires additional research for complete characterization.
Economic Decision Framework
Abrasive blast cleaning adds $0.10-0.30 per pound to galvanizing costs, varying by part geometry, production volume, and regional market conditions. Cost justification requires analyzing potential quality issues against preventive processing expense.
Scenarios justifying pre-galvanizing blast cleaning:
- Low-silicon steel producing consistently thin coatings
- Cost of blast cleaning < cost of rejected parts failing thickness requirements
- Especially critical for projects specifying thickness above ASTM minimums
- Highly reactive steel in thick sections or complex assemblies
- Cost of blast cleaning < cost of coating repair/remediation for flaking
- Prevents field performance issues and warranty claims
- Reduces handling damage risk during fabrication and installation
- Projects with stringent aesthetic requirements
- Blast cleaning improves coating uniformity and appearance
- Surface profile reduces spangling visibility on low-silicon steel
- Justifiable when architectural exposed applications demand premium appearance
Scenarios where blast cleaning adds unnecessary cost:
- Optimal chemistry steel (Si 0.15-0.28%)
- Minimal thickness effect observed
- Standard galvanizing process produces acceptable coatings
- Better investment: verify steel chemistry during procurement
- Thin materials with brief immersion times
- Insufficient kettle time for blast cleaning effects to manifest
- Sheet, plate <1/4", and light structural shapes typically unaffected
- Projects without thickness control concerns
- Steel chemistry already produces coatings within acceptable range
- Blast cleaning provides no performance benefit
Alternative Coating Thickness Control Methods
Pre-galvanizing blast cleaning represents one approach within a comprehensive thickness management strategy:
Reducing coating thickness on reactive steel:
- Steel specification optimization: Specify silicon content 0.15-0.25% to avoid high reactivity
- Kettle chemistry management: Aluminum additions to zinc bath can moderate reactive steel behavior
- Improved venting/drainage: Reduce immersion time through optimized hole size and placement
- Quench technique variation: Accelerated cooling may limit coating growth
Increasing coating thickness on low-silicon steel:
- Steel specification adjustment: Accept silicon content 0.15-0.25% rather than <0.04%
- Extended immersion time: Allow additional kettle time for diffusion coating growth
- Multiple dipping: Sequential galvanizing passes build coating thickness (specialty application)
Ongoing Research Directions
The 2021 AGA study established baseline data across major steel categories, but several areas warrant continued investigation:
- Upper silicon limits: Define silicon content threshold where blast cleaning loses effectiveness
- Recycled steel variability: Characterize coating response in steels with complex chemistry from scrap content
- Blast media optimization: Compare steel grit vs. aluminum oxide vs. other media for thickness control
- Profile depth effects: Quantify relationship between profile height and coating thickness response
- Long-term durability: Assess whether blasted surface coatings perform differently in atmospheric service
Specification Guidance
For projects considering pre-galvanizing abrasive blast cleaning:
When specifying blast cleaning for coating enhancement (low-silicon steel):
- Require steel mill test reports documenting silicon content <0.04%
- Specify minimum coating thickness targets above ASTM A123 requirements
- Reference SSPC-SP 6 for blast cleanliness guidance
- Allow galvanizer to optimize profile depth (typically 2-3 mils adequate)
When specifying blast cleaning for thickness reduction (reactive steel):
- Identify steel silicon content >0.25% through mill certifications
- Recognize that extended immersion amplifies blast cleaning effectiveness
- Consider complementary strategies: improved venting, kettle chemistry adjustments
- Establish maximum coating thickness requirements if flaking concerns exist
General specification principles:
- Do not blanket-specify blast cleaning without steel chemistry justification
- Optimal steel selection (Si 0.15-0.25%) eliminates most thickness control issues
- Verify galvanizer capability to perform in-house blast cleaning (not all facilities equipped)
- Document rationale for blast cleaning specification to enable value engineering discussions
The American Galvanizers Association's 2021 research quantifies how abrasive blast cleaning before hot-dip galvanizing affects coating thickness development across the steel reactivity spectrum. Results demonstrate significant coating enhancement on low-silicon steels and meaningful thickness reduction on highly reactive compositions with extended immersion times, while minimal effects occur on optimal chemistry steels. These findings enable engineers to strategically apply blast cleaning where technical benefits justify additional processing costs—particularly for low-silicon structural steel requiring thickness enhancement and highly reactive steel in thick sections prone to excessive coating formation. However, the most cost-effective approach to coating thickness control remains proper steel specification, selecting compositions in the 0.15-0.25% silicon range that naturally produce optimal coatings without supplementary surface preparation. Pre-galvanizing blast cleaning serves as a valuable corrective tool when steel chemistry cannot be controlled, but should not substitute for fundamental material selection that prevents thickness issues at the source. Visit the original AGA resource on abrasive blasting on HDG to learn more.
