Hot dip galvanizing requires complete immersion of steel into molten zinc. For the coating to form properly, molten zinc must freely contact all steel surfaces. When steel components are designed with overlapping or tightly contacting surfaces, challenges can arise during the galvanizing process.
Overlapping areas, particularly when unsealed, can trap air, cleaning solutions, or moisture. These trapped pockets may prevent proper cleaning, restrict zinc flow, or create long term corrosion concerns if not properly addressed.
This article examines why overlapping joints require careful design attention and how fabrication practices influence galvanizing outcomes.
Why Overlapping Areas Create Challenges
When two steel surfaces are in direct contact or closely spaced without sealing, several issues may occur:
- Cleaning solutions may not penetrate fully during surface preparation
- Air may be trapped during immersion
- Molten zinc may not reach internal surfaces
- Residual chemicals may remain after processing
Because galvanizing depends on proper surface preparation before immersion, incomplete cleaning in overlapping areas can result in uncoated regions.
Additionally, small gaps can act as crevices, which are more susceptible to corrosion if not properly sealed.
Surface Preparation Limitations
During the galvanizing process, steel undergoes multiple preparation stages including alkaline cleaning, rinsing, acid pickling, and flux application before immersion.
If overlapping joints are tightly contacting and unsealed:
- Pickling acid may not reach internal surfaces
- Flux may not coat hidden steel
- Zinc may not flow completely into the joint
This can lead to uncoated interior surfaces that are difficult or impossible to inspect after galvanizing.
For this reason, design practices that eliminate tight, inaccessible crevices are strongly recommended.
Sealed Versus Unsealed Overlapping Joints
Overlapping steel can be acceptable if properly sealed.
Fully welded lap joints allow:
- Complete cleaning
- Uniform immersion
- Elimination of trapped air pockets
- Reduced risk of crevice corrosion
Unsealed overlapping areas, particularly when intermittently welded or left open at edges, can create concealed cavities.
If a cavity is completely sealed without venting, internal pressure buildup during immersion can also pose safety risks.
Proper fabrication planning must consider both corrosion and safety implications.
Venting and Drainage Considerations
Even when overlapping areas are sealed, venting and drainage design remains critical.
If any cavity is created by overlapping plates or stiffeners, adequate vent holes must allow:
- Air to escape during immersion
- Molten zinc to enter and exit freely
- Cleaning solutions to circulate
Improper venting can result in:
- Incomplete coating
- Surface defects
- Safety hazards
Design drawings should clearly indicate venting details when overlapping components create enclosed spaces.
Long Term Corrosion Concerns
Overlapping joints can create crevice environments where moisture may be retained.
In atmospheric exposure, galvanized coatings provide predictable protection. However, if moisture becomes trapped between contacting surfaces with limited zinc coverage, corrosion risk increases.
Best practices include:
- Fully welding overlapping joints where possible
- Avoiding intermittent welds that leave unsealed pockets
- Designing joints to eliminate tight crevices
Attention to joint detail during fabrication significantly improves long term durability.
Design Recommendations for Engineers
To minimize galvanizing complications related to overlapping steel:
- Avoid tightly contacting surfaces that cannot be cleaned
- Fully seal lap joints through continuous welding
- Provide adequate venting for any enclosed cavities
- Consider alternative connection designs where feasible
Early coordination during design prevents coating issues later.
We routinely review fabrication drawings to identify potential overlapping area concerns before galvanizing begins. If you would like us to review your design details for galvanizing compatibility, please reach out through our contact page.
Proactive discussion reduces rework, improves coating quality, and enhances long term performance.
Overlapping steel surfaces can interfere with proper hot dip galvanizing if not properly designed and sealed. Tight crevices may restrict cleaning and zinc flow, while unvented cavities present both coating and safety risks. Fully welded joints and adequate venting ensure coating continuity and structural durability.
For the original technical reference, review:
https://galvanizeit.org/knowledgebase/article/overlapping-areas
Frequently Asked Questions About Overlapping Areas in Galvanizing
Are overlapping steel plates acceptable for hot dip galvanizing?
Yes, but they must be properly designed. Fully sealed and welded lap joints are generally acceptable, while tight unsealed crevices can cause coating issues.
Why are unsealed lap joints problematic?
Unsealed joints can trap cleaning solutions or air, preventing proper zinc contact and creating potential corrosion sites.
Do overlapping areas require vent holes?
If overlapping components create enclosed cavities, venting is required to allow air escape and zinc flow during immersion.
Can overlapping joints cause corrosion after galvanizing?
Yes. Crevice environments between contacting surfaces may retain moisture and increase corrosion risk if not properly sealed.
Should overlapping details be reviewed before fabrication?
Yes. Early review helps ensure proper cleaning, venting, and long term coating performance.
Is intermittent welding acceptable for overlapping joints?
Intermittent welding that leaves unsealed cavities can create coating and corrosion issues. Continuous welding is generally preferred.
What happens if zinc cannot reach an overlapping area?
The steel in that area may remain uncoated, leading to potential corrosion initiation and non compliance with coating continuity requirements.
