The Low-Reactivity Steel Challenge
Hot-dip galvanized coating thickness results from zinc-iron metallurgical reactions occurring at the steel-zinc interface during immersion in molten zinc. Reaction kinetics depend heavily on steel chemistry, with certain alloying elements—particularly silicon and phosphorus—acting as catalysts that accelerate alloy layer formation and increase coating thickness. However, steels with extremely low concentrations of these catalytic elements produce minimal zinc-iron reactions, generating thin coatings that may struggle to meet minimum thickness requirements established by ASTM A123 and ASTM A153. Understanding low-reactivity steel behavior, available mitigation strategies, and the limitations of extended acid pickling approaches enables informed decisions when confronting coating thickness challenges.
Steel Chemistry and Coating Thickness Relationships
The relationship between steel composition and galvanized coating thickness is well-documented through decades of research and practical experience:
Silicon Content Effects
Silicon content exerts the most dramatic influence on coating thickness through its role in zinc-iron reaction kinetics:
Ultra-Low Silicon (<0.02% Si): Steel with silicon content below 0.02% produces minimal zinc-iron alloying reactions. The coating consists primarily of a thin layer of zinc-iron alloys topped with pure zinc adhering to the surface. Total coating thickness typically ranges from 1.5 to 2.5 mils (38-64 micrometers), often falling below ASTM A123 minimum requirements for many steel thickness categories.
Low-Normal Silicon (0.02-0.04% Si): Slightly elevated silicon produces moderately increased reaction rates, generally achieving coating thickness in the 2.0-3.5 mils (51-89 micrometers) range that meets most specifications.
Sandelin Range (0.04-0.13% Si): This intermediate range produces unpredictable, variable coating thickness with generally poor performance. Historically problematic, modern steel production often avoids this composition range.
Sebisty Range (0.13-0.28% Si): High silicon content dramatically accelerates zinc-iron reactions, producing very thick coatings (5-12+ mils) with characteristic matte gray appearance from extensive alloy layer formation.
High Silicon (>0.28% Si): Silicon content above this threshold moderates reaction rates, returning to controlled coating formation similar to low-silicon behavior.
Phosphorus Interaction
Phosphorus content below 0.02% similarly reduces zinc-iron reaction catalysis. The combined effect of both low silicon and low phosphorus produces the most challenging conditions for achieving adequate coating thickness.
Aluminum-Killed Steel
Steel deoxidized with aluminum during steelmaking—termed "aluminum-killed steel"—exhibits particularly low reactivity during galvanizing. The aluminum deoxidation process removes dissolved oxygen, producing clean steel with excellent mechanical properties but minimal silicon content. This steel chemistry combination consistently produces thin galvanized coatings.
Aluminum-killed steels are widely produced for applications requiring superior formability, weldability, and surface quality. Their prevalence in modern steel production means galvanizers frequently encounter coating thickness challenges with material ordered for unrelated fabrication properties rather than galvanizing performance.
ASTM A385: Guidance for Low-Reactivity Steel
ASTM A385, "Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip)," acknowledges low-reactivity steel challenges and provides recommendations for optimizing galvanizing outcomes:
Recommended Steel Chemistry Ranges
A385 establishes optimal composition ranges for steel intended for hot-dip galvanizing:
- Carbon: 0.15-0.25%
- Manganese: 0.60-1.20%
- Silicon: 0.03-0.06% or 0.15-0.22%
- Phosphorus: 0.04-0.10%
These ranges avoid problematic compositions including ultra-low silicon/phosphorus and the unpredictable Sandelin range while leveraging silicon's beneficial catalytic effects.
Options for Low-Coating Situations
When low-reactivity steel produces coatings below specification minimums, A385 recommends galvanizers and purchasers collaboratively determine appropriate action:
Option 1: Accept Lower Coating Thickness
For many applications, coating thickness below ASTM A123 minimums still provides adequate corrosion protection. Service life is proportional to coating thickness, so a 2.0-mil coating instead of 3.0 mils delivers approximately two-thirds the protection duration.
Appropriate Scenarios:
- Benign exposure environments with low corrosivity
- Applications accepting shorter service intervals between maintenance
- Interior installations with minimal atmospheric exposure
- Temporary structures with limited design life requirements
Option 2: Duplex System Application
Applying paint or powder coating over the thin galvanized substrate creates a duplex system providing enhanced protection. The organic topcoat serves as primary barrier protection while the galvanized layer provides cathodic protection at defects and extends system life.
Advantages:
- Compensates for thin galvanized coating through supplemental protection
- Provides aesthetic customization through color selection
- Delivers extended service life exceeding either coating system individually
Considerations:
- Increased initial cost for topcoat application
- Requires appropriate surface preparation
- May require periodic topcoat maintenance
Option 3: Abrasive Blast Cleaning Before Galvanizing
Pre-galvanizing abrasive blasting removes mill scale while roughening the steel surface, increasing effective surface area for zinc-iron reactions and typically enhancing coating thickness by 0.5-1.5 mils.
Mechanism: Surface roughness increases microscopic surface area where zinc-iron reactions occur. The increased surface area enables more extensive alloy layer formation despite low-reactivity chemistry.
Advantages:
- Can achieve 20-40% coating thickness increases
- Simultaneously ensures complete mill scale removal
- Applicable to most steel geometries
Considerations:
- Additional processing cost and time
- Requires blast cleaning facilities and expertise
- May affect dimensional tolerances or surface finish requirements
- Generates abrasive waste requiring disposal
Option 4: Extended Pickling in Sulfuric Acid
Prolonged immersion in sulfuric acid beyond the duration necessary for scale removal roughens the steel surface through controlled chemical attack, potentially increasing subsequent coating thickness.
This option—the focus of this article—presents the most significant implementation challenges and quality risks among the A385 alternatives.
Extended Pickling Mechanism and Theory
Normal pickling operations remove mill scale and rust from steel surfaces through chemical dissolution, exposing clean metallic iron for subsequent galvanizing. Pickling duration is calibrated to achieve complete scale removal without excessive steel base metal attack.
Surface Roughening Through Acid Attack
Extended pickling—also termed "overpickling" or "overchemical cleaning"—deliberately prolongs acid exposure beyond scale removal completion, allowing acid to attack the steel substrate itself. This controlled corrosive attack produces surface roughness through:
Preferential Grain Boundary Attack: Acid attacks grain boundaries and crystallographic defects preferentially compared to grain interiors, creating microscopic surface texture.
Non-Uniform Dissolution: Variations in local steel chemistry, crystal orientation, and surface condition produce non-uniform dissolution rates, generating surface irregularity.
Increased Surface Area: The roughened surface provides greater total area for zinc-iron reactions during subsequent galvanizing, potentially increasing coating thickness.
Surface Activation: Acid attack may remove surface layers with reduced reactivity, exposing fresh steel with enhanced alloying propensity.
Theoretical Coating Thickness Enhancement
Research and anecdotal experience suggest extended pickling can increase coating thickness by approximately 0.3-0.8 mils (8-20 micrometers) compared to normally pickled low-reactivity steel. This enhancement, while modest, may prove sufficient to achieve minimum specification compliance for articles marginally below requirements.
However, the actual benefit varies dramatically based on:
- Initial steel chemistry and reactivity
- Acid concentration and temperature
- Extension duration beyond normal pickling
- Steel surface condition and cleanliness
- Subsequent galvanizing parameters
The unpredictable nature of coating thickness response to extended pickling represents a significant practical limitation.
Process Implementation Challenges
Despite appearing simple in principle, extended pickling presents substantial implementation obstacles that limit its practical application:
Limited Sulfuric Acid Availability
Approximately 40% of North American galvanizing facilities use sulfuric acid for pickling, with the remaining 60% employing hydrochloric acid. Extended pickling specifically requires sulfuric acid due to its surface etching characteristics; hydrochloric acid extended exposure produces different surface effects less beneficial for coating enhancement.
Implication: More than half of galvanizers cannot implement extended pickling regardless of willingness, immediately eliminating this option for substantial portions of the industry.
Acid Inhibitor Complications
Many sulfuric acid pickling operations incorporate acid inhibitors—chemical additives that suppress steel base metal attack while maintaining scale removal effectiveness. Inhibitors improve process control and economics by:
- Reducing acid consumption through decreased steel dissolution
- Minimizing hydrogen evolution and associated safety concerns
- Extending pickling bath life by reducing dissolved iron accumulation
- Preventing excessive surface roughening during normal operations
Extended Pickling Conflict: Acid inhibitors directly counteract the surface roughening mechanism extended pickling seeks to achieve. Facilities using inhibited acid cannot effectively implement extended pickling without removing inhibitors—a change requiring complete acid bath replacement or switching to non-inhibited acid systems.
The operational disruption and cost of such changes make extended pickling impractical for many facilities employing inhibitors.
Lack of Process Control Precision
Extended pickling requires careful control of immersion duration to achieve beneficial roughening without excessive steel attack. However, establishing optimal extension time involves multiple variables:
Material-Dependent Responses: Different steel heats respond variably to extended pickling based on chemistry differences, surface condition, and previous processing history.
Bath Condition Variations: Acid concentration, temperature, contamination level, and dissolved iron content affect attack rates, causing day-to-day variability.
Geometry Effects: Thin sections may suffer excessive attack while heavy sections receive inadequate roughening when using uniform immersion duration.
Lack of Visual Indicators: Unlike normal pickling where mill scale removal provides clear endpoint indication, optimal extended pickling duration has no obvious visual indicator, requiring trial-and-error experimentation.
This unpredictability creates significant risk of inadequate treatment (no coating improvement) or excessive treatment (steel damage).
Quality and Aesthetic Consequences
Even when successfully increasing coating thickness, extended pickling produces several potentially problematic quality effects:
Excessive Surface Roughness
The surface roughening beneficial for coating thickness enhancement creates tactile and visual texture that may prove unacceptable for certain applications:
Fit-Up Complications: Roughened surfaces on components requiring precise assembly—threaded connections, bearing surfaces, close-tolerance interfaces—can interfere with proper fit-up or cause binding during assembly.
Tactile Concerns: Handrails, architectural elements, or products involving frequent human contact present uncomfortable or abrasive surfaces after extended pickling and galvanizing.
Aesthetic Objections: The rough surface texture creates a coarse, irregular galvanized finish contrasting with the relatively smooth appearance of normally pickled articles. This appearance may not meet aesthetic requirements for architectural or product applications.
Coating Adhesion: While moderate roughness enhances paint adhesion in duplex systems, excessive roughness can trap air or contaminants, potentially compromising topcoat quality.
Uneven Appearance and Thickness Distribution
Extended pickling rarely produces uniform surface roughening across complex geometries:
Localized Variation: Areas with different flow patterns, concentration gradients, or temperature exposure during pickling receive varying degrees of surface attack, creating appearance irregularities.
Edge Effects: Sharp edges, corners, and thin sections experience preferential acid attack compared to flat surfaces or heavy sections, producing thickness variations and appearance differences across article surfaces.
Coating Texture Differences: Non-uniform roughening translates to non-uniform coating distribution and appearance after galvanizing, with some areas showing smooth bright finish while others display rough texture.
Discoloration Patterns: Differential pickling attack can create color variations in the final galvanized coating even when coating thickness is adequate.
For applications requiring visual uniformity—architectural metalwork, consumer products, decorative elements—these appearance variations may render extended pickling unacceptable regardless of coating thickness improvement.
Risk of Excessive Steel Loss
The most severe risk involves prolonged acid exposure crossing the threshold from beneficial roughening to destructive steel removal:
Dimensional Impact: Excessive pickling can remove sufficient steel to affect dimensional tolerances, particularly on thin sections or precision-machined surfaces.
Structural Weakening: Aggressive acid attack may reduce cross-sectional area sufficiently to compromise load-carrying capacity, especially for thin-walled tubular products or fine mesh materials.
Complete Perforation: In extreme cases, extended pickling can perforate thin sections entirely, creating holes that render articles unusable.
Thread Damage: Threads subject to extended pickling may experience sufficient material loss to degrade thread form and reduce connection strength.
The irreversible nature of acid attack means excessive pickling cannot be remedied—damaged articles must be scrapped and replaced.
Mixed Assembly Complications
Production lots frequently contain assemblies fabricated from multiple steel components with different chemistries, thicknesses, or surface conditions. Extended pickling proves particularly problematic for such mixed assemblies:
Differential Response
Different steel chemistries respond variably to extended acid exposure:
High-Reactivity Components: Steel with normal or elevated silicon content requires no extended pickling and may suffer excessive roughening or damage if subjected to extended immersion appropriate for low-reactivity steel.
Fabricated Surfaces: Welded zones, machined surfaces, or formed regions may respond differently to acid attack compared to as-rolled surfaces, creating inconsistent results.
Varying Thickness Sections: Thin sections lose material faster than heavy sections during extended pickling, potentially causing thin component damage while thicker components receive beneficial treatment.
All-or-Nothing Processing
Galvanizing operates on a batch processing model where all articles in a galvanizing load undergo identical chemical cleaning. When a load contains mixed steel chemistries requiring different pickling durations:
Conservative Approach: Using standard pickling duration protects all components from damage but fails to enhance low-reactivity steel coating thickness.
Aggressive Approach: Extending pickling to benefit low-reactivity components risks damaging normal-reactivity steel, creating quality problems for the majority of the assembly.
No Optimal Compromise: Mixed assemblies generally have no pickling duration satisfying all components simultaneously.
The practical solution often involves avoiding extended pickling entirely for mixed assemblies, accepting thin coatings on low-reactivity components while protecting overall assembly integrity.
Alternative Approaches for Low-Reactivity Steel
Given extended pickling's limitations, alternative strategies often provide more reliable solutions:
Prospective Steel Selection
The most effective approach involves specifying appropriate steel chemistry before fabrication:
Request Silicon Content: When ordering steel for structures requiring galvanizing, specify silicon content within ASTM A385 optimal ranges (0.03-0.06% or 0.15-0.22%).
Avoid Aluminum-Killed Steel: Specify steel deoxidation methods other than aluminum killing, or accept that aluminum-killed steel may produce thin coatings requiring supplemental protection.
Review Mill Test Reports: Obtain and review steel chemistry certifications before fabrication begins, allowing material substitution if composition proves unsuitable for galvanizing.
Long-Term Supplier Relationships: Develop relationships with steel suppliers understanding galvanizing requirements and consistently providing appropriate chemistries.
Modified Galvanizing Parameters
Galvanizers can sometimes enhance coating thickness through process adjustments:
Extended Immersion Time: Increasing zinc bath immersion duration by 30-100% compared to standard practice can incrementally increase coating thickness on low-reactivity steel without requiring extended pickling.
Elevated Bath Temperature: Operating galvanizing kettles at the upper end of acceptable temperature ranges (850-860°F versus 820-840°F) accelerates zinc-iron reactions, potentially compensating partially for low steel reactivity.
Multiple Immersion: Some facilities can perform sequential galvanizing immersions, removing articles briefly between immersions to cool, then re-immersing to build additional coating thickness.
Specialized Bath Chemistry: Zinc bath additions or modifications can sometimes enhance reactivity with low-silicon steel, though this approach requires significant metallurgical expertise.
Mechanical Surface Preparation
Pre-galvanizing abrasive blasting provides more controlled and predictable surface roughening than extended pickling:
Better Process Control: Blast parameters (media type, pressure, duration) can be adjusted precisely for different components and thicknesses.
Uniform Results: Properly executed blasting produces consistent surface roughness across complex geometries.
No Damage Risk: Unlike acid attack, blasting removes minimal base metal and poses little risk of dimensional or structural compromise.
Predictable Outcomes: The coating thickness enhancement from blasting is more consistent and reliable than extended pickling results.
Communication and Expectations Management
Successful management of low-reactivity steel galvanizing requires transparent communication among all stakeholders:
Early Disclosure
Fabricator to Galvanizer: When steel chemistries are known to be marginal for galvanizing (from mill test reports), inform the galvanizer early in project planning. This advance notice allows:
- Discussion of processing options and limitations
- Trial galvanizing of sample pieces if appropriate
- Development of contingency plans before production commitment
- Realistic schedule expectations accounting for potential rework or alternative processing
Galvanizer to Customer: If coating thickness concerns are identified during receiving inspection or initial processing trials, immediately notify customers rather than proceeding with questionable results. Early communication enables collaborative problem-solving.
Realistic Capability Representation
Galvanizer Capabilities: Galvanizers should accurately represent their facility capabilities regarding extended pickling:
- Acid type in use (sulfuric versus hydrochloric)
- Inhibitor status and flexibility for adjustment
- Experience and success rate with extended pickling
- Alternative approaches available at the facility
Commitment Limitations: If extended pickling capabilities are unavailable or unreliable, clearly communicate this limitation rather than committing to uncertain outcomes.
Quality Expectation Alignment
Appearance Tolerance: Discuss and establish acceptable appearance standards considering that extended pickling produces rougher, less uniform coatings than normal processing.
Dimensional Acceptance: Clarify whether dimensional changes from extended pickling are acceptable and establish inspection criteria verifying acceptability.
Coating Thickness Priorities: Determine whether minimum coating thickness compliance takes priority over appearance considerations, or if appearance uniformity is paramount even if coating thickness remains marginally low.
Economic Considerations
Extended pickling's economic profile involves several factors:
Direct Costs
Acid Consumption: Extended immersion consumes more acid through steel dissolution, increasing chemical costs.
Processing Time: Longer pickling duration reduces facility throughput, potentially requiring premium pricing to offset reduced capacity.
Trial Expenses: Establishing optimal extended pickling parameters may require test runs and sample pieces, adding project cost.
Quality Risk Costs: Potential for damaged articles requiring rework or replacement creates financial risk.
Indirect Costs
Customer Dissatisfaction: Appearance issues or fit-up problems from extended pickling can damage customer relationships and reputation.
Technical Support Time: Extended pickling complications require additional engineering and technical support resources.
Rework Risk: If extended pickling proves unsuccessful, articles may require reprocessing with alternative approaches or complete project restart.
Value Comparison
Compared to alternative approaches:
Versus Proper Steel Selection: Specifying appropriate steel chemistry eliminates coating thickness concerns at minimal cost premium (steel price differences are typically 1-3%).
Versus Abrasive Blasting: Pre-galvanizing blast cleaning provides more reliable results with typical cost of $0.50-2.00 per pound compared to extended pickling's uncertain benefits.
Versus Duplex Systems: Topcoating thin galvanized substrates provides superior total protection and appearance control compared to attempting coating thickness enhancement through extended pickling.
Regulatory and Environmental Considerations
Extended pickling has environmental implications:
Increased Acid Waste
Higher steel dissolution increases dissolved iron and metal contamination in pickling baths, accelerating bath degradation and disposal frequency.
Air Emissions
Extended pickling generates increased hydrogen gas evolution and acid mist emissions, requiring enhanced ventilation and emissions control.
Waste Treatment
Spent pickling solutions containing elevated dissolved metals require proper neutralization and disposal according to environmental regulations.
Industry Best Practice Recommendations
Based on extensive industry experience, the following practices are recommended:
For Specifiers and Designers
- Specify steel chemistry per ASTM A385 recommendations when galvanizing is intended
- Review mill test reports during procurement to verify compliance
- Avoid aluminum-killed steel for galvanized applications unless thin coatings are acceptable
- Consider duplex systems proactively for low-reactivity steel rather than attempting enhancement
- Establish clear coating thickness and appearance priorities in specifications
For Fabricators
- Communicate steel chemistry information to galvanizers early in project timelines
- Maintain records of supplier steel chemistries and galvanizing performance
- Work with steel suppliers to source appropriate chemistries for galvanizing applications
- Budget for potential supplemental protection (duplex systems) when low-reactivity steel is unavoidable
For Galvanizers
- Clearly communicate facility capabilities regarding extended pickling
- Recommend alternative approaches (blasting, duplex systems, modified parameters) before agreeing to uncertain extended pickling
- Document extended pickling parameters and results to build institutional knowledge
- Establish trial piece protocols before committing production lots to experimental processing
- Maintain conservative approach prioritizing quality over marginal coating thickness gains
Extended sulfuric acid pickling represents one of several strategies for addressing coating thickness challenges with low-reactivity steel containing ultra-low silicon and phosphorus or produced through aluminum-killing deoxidation. While theoretically capable of increasing coating thickness through controlled surface roughening, the approach faces substantial practical limitations including restricted availability (only 40% of galvanizers use sulfuric acid), acid inhibitor complications, unpredictable results requiring trial-and-error optimization, and significant quality risks including excessive surface roughness, appearance irregularities, and potential steel damage. Mixed assemblies containing steel with varying chemistries present particular challenges as uniform extended pickling cannot simultaneously benefit low-reactivity components without damaging normal-reactivity steel. Alternative approaches—particularly prospective steel specification to avoid problematic chemistries, pre-galvanizing abrasive blasting, modified galvanizing parameters, or duplex system application over thin coatings—generally provide more reliable and predictable outcomes than extended pickling. When extended pickling is contemplated, transparent communication among galvanizers, fabricators, and customers regarding capabilities, limitations, and quality expectations is essential. However, the optimal strategy remains proper steel selection during procurement, following ASTM A385 chemistry recommendations to ensure adequate coating thickness through normal galvanizing processes without requiring extraordinary enhancement measures carrying uncertain outcomes and significant quality risks. See the original AGA resource here.
