IMMERSION PRETREATMENT LINE WITH 7-STAGE DIP TANKS

Immersion Pretreatment Line with 7-Stage Dip Tanks | Delivered to an HVAC Component Manufacturer in S******t A**

01　THE CHALLENGE

CASE STUDY · COATTECH PROJECTS — Dated Oct 8, 2026

IMMERSION PRETREATMENT LINE WITH 7-STAGE DIP TANKS

Engineered for an HVAC Component Manufacturer in S******t A**

Project at a glance

Project: Immersion pretreatment line with 7 dip tanks and overhead conveyor handling

Client: H* C*****g Industry Co. (name partially withheld at the client's request)

Location: S******t A** — designed, manufactured and shipped from CoatTech factory, China

Industry: HVAC and air conditioning components

Workpiece: Perforated and stamped sheet metal components with internal cavities and blind holes

Chemistry: Alkaline degreasing → rinse → de-oxidising → rinse → zinc phosphating → rinse → passivation

Handling: Overhead chain conveyor with dedicated drainage fixtures

Scope: Process design, tank fabrication, conveyor integration, installation guidance and commissioning support

01 THE CHALLENGE

The client manufactures perforated, stamped sheet metal components with internal cavities and blind holes. Spray pretreatment could not deliver chemistry into those recesses, so untreated areas survived to the coating stage and later showed up as adhesion failures and rework.

Two further problems compounded it. Loads were handled in batches by hand, so treatment time varied from one load to the next. And chemistry carried over between stages as loads were lifted and moved, unbalancing bath concentration and shortening bath service life.

02　THE SOLUTION　

02 THE SOLUTION

CoatTech supplied an immersion line built on full submersion rather than spray. Every component is hung on a dedicated fixture and carried by an overhead chain conveyor through seven stages, so each load receives identical treatment — same sequence, same dwell time, every cycle.

Fixture geometry was designed around two requirements. Fixtures hold parts fully below the liquid line even where hollow sections trap air, and they are set at an angle so chemistry drains back into the tank on lift-out, which is what controls carry-over between stages.

Tanks were fabricated in SUS316 stainless steel with welded seams — specified over SUS304 because the phosphating stage runs heated and acidic. Each tank has circulation sized to its volume for uniform temperature and concentration, and heating to hold the bath inside its working window. Yellow and black perimeter railing, anti-slip walkways, emergency stops and lip extraction were included for operator safety.

The chemistry sequence was specified as alkaline degreasing, rinse, de-oxidising, rinse, zinc phosphating, rinse and passivation. Zinc phosphating was selected because the components are steel and the coating specification called for corrosion performance beyond what a silane or zirconium conversion layer alone would deliver. For clients working to lower-temperature or chrome-free requirements, CoatTech also engineers zirconium and silane sequences on the same tank platform.

03　RESULTS　

03 RESULTS

Rework rate reduced from approximately 8% to 1.5%, as chemistry now reaches the cavities and blind holes that a spray pattern could not.

Bath service life extended by around 40% through reduced carry-over between stages.

Operators required on the pretreatment section reduced from 6 to 2.

Treatment consistency stabilised across shifts, because dwell time is set by the conveyor rather than by the operator.

04　TECHNICAL SPECIFICATIONS

05　FAQ

Q: Why choose immersion pretreatment instead of a spray tunnel?

A: Immersion is the right choice when components have internal cavities, blind holes, deep recesses or dense perforations that a spray pattern cannot reach. Spray is better for flat, simple parts at high continuous volume, or where floor space is limited. For parts like these HVAC components, full submersion is the only way to guarantee chemistry reaches every surface the coating must adhere to.

Q: What is zinc phosphating and when is it used?

A: Zinc phosphating is a conversion coating process that deposits a thin crystalline phosphate layer on steel, improving paint adhesion and corrosion resistance. It is specified where the coating standard demands corrosion performance beyond what a silane or zirconium layer alone provides. Silane and zirconium treatments are lower-temperature, chrome-free alternatives, typically used where environmental rules or energy cost rule out a heated phosphate bath.

Q: How is chemical carry-over between stages controlled?

A: Fixture geometry is set at an angle so chemistry drains back into the tank as the load lifts out. This keeps bath concentration stable, which directly extends bath service life — in this project by around 40%.

Q: Can one line run both immersion and spray?

A: Yes. A common configuration sprays the outer surfaces for bulk cleaning, then immerses for the conversion coating stage so chemistry reaches cavities the spray missed. CoatTech engineers hybrid sequences where the workpiece geometry calls for it.

Q: Which stainless steel grade should an immersion tank use?

A: SUS304 suits standard alkaline degreasing and silane chemistry. SUS316 is specified for heated acidic stages such as zinc phosphating, or for high chloride content, and gives longer service life in continuous operation.

06　WHY COATTECH

WHY COATTECH

Process-first engineering — we specify immersion, spray or a hybrid from the workpiece geometry, not from a catalogue.

One supplier, one responsibility — pretreatment, conveyor, drying and coating engineered and delivered as one line.

Export-ready — CE-compliant construction, containerised shipping, installation drawings and remote commissioning support.

Proven across markets — operating lines in Europe, the Middle East, Africa, Russia and Southeast Asia.

Talk to our engineering team

Send us your workpiece drawings, substrate material and required corrosion performance. We will tell you whether immersion, spray or a hybrid sequence suits your parts, and return a tank layout with a budgetary proposal.