In numbers
| Quantity | Value | Where it comes from |
|---|---|---|
| Iron phosphate as a one-stage cleaner-coater | 40 to 60 C, 0.4 to 0.8 g/m² | Henkel BONDERITE M-FE 3990-1 data sheet, October 2019 |
| Spray iron phosphate | 40 to 60 C, 2 to 3 min, 1 to 2 bar | Henkel BONDERITE M-FE 3803 data sheet, August 2020 |
| Zirconium thin-film | 20 to 40 C, 20 to 120 s, 0.5 to 1.5 bar | Henkel BONDERITE M-NT 1 data sheet, March 2025 |
| Highest corrosivity category for iron phosphate on steel | C3 | QUALISTEELCOAT v5.0.1, valid from 1 January 2026 |
| Minimum etch on architectural aluminium | 1.0 g/m² | QUALICOAT 26th edition, section 2.5 |
| Line speed of our documented complete plant | 1.5 m/min | Adencoat line |
The stage count follows the chemistry and the corrosivity category
The number of pretreatment stages on a powder coating line follows from two choices: the conversion chemistry and the corrosivity category the coated part has to reach. Degreasing and rinsing are the minimum on any line. Everything added after that serves adhesion and corrosion resistance.
Iron phosphate is the shortest route. Henkel's BONDERITE M-FE 3990-1 degreases and phosphates in a single stage at 40 to 60 C and deposits 0.4 to 0.8 g/m², according to its 2019 data sheet. Zinc phosphate is longer. One supplier's white paper states that the process "must involve multiple stages", with cleaning and conditioning before the phosphating bath, and the bath itself runs at 55 C or above (Products Finishing). Zirconium thin-film chemistry runs cold, but one supplier states that its silane-based Oxsilan process needs "at least 5 stages".
A pretreatment proposal therefore starts from the class on the customer's drawing.
QUALISTEELCOAT v5.0.1 caps iron phosphate at C3 and a single coat at C2
QUALISTEELCOAT v5.0.1, valid from 1 January 2026, is the primary text that ties pretreatment on steel to a corrosivity category. Its wording is direct: "C3 is the maximum corrosivity category for chemical pre-treatment based on iron phosphate or without forming a conversion layer". A one-layer powder system on steel is limited to C2.
The same specification sets the neutral salt spray duration for each class and limits the time between pretreatment and powder. Under section 3.6.1 the oven is checked at four points, weekly for classes C3 to C5.
| Category | Neutral salt spray | Pretreatment to coating | Iron phosphate allowed | One-layer powder allowed |
|---|---|---|---|---|
| C2 | 240 h | 24 h or less | Yes | Yes |
| C3 | 480 h | 24 h or less | Yes | No |
| C4 | 720 h | 8 h or less | No | No |
| C5 | 1,440 h | 8 h or less | No | No |
A tunnel built for iron phosphate therefore serves C2 and C3 work. C4 and C5 work needs a conversion coat that forms a layer the specification accepts, and with it a longer sequence.
Galvanised steel adds a three-hour limit after sweep blasting
Hot-dip galvanised parts carry a time limit of their own: QUALISTEELCOAT v5.0.1 requires them to be coated within 3 h of sweep blasting, on top of the general limit of 24 h from pretreatment to coating for C1 to C3 and 8 h for C4 and C5. EN 13438 is the product standard for powder coatings on galvanised steel.
The three-hour limit is a layout and scheduling matter as much as a chemistry matter. Blasting, pretreatment and booth have to sit in a sequence the parts pass through without a buffer store in between.
The C3 sentence quoted above is written about the pretreatment chemistry, so we read it as applying to galvanised parts pretreated with iron phosphate as well. Which thin-film systems qualify galvanised parts for C4 or C5 is set in Annex 3 of the specification, which we have not reviewed, so this post makes no claim on it.
Aluminium needs an etch, a conversion coat and a deionised final rinse
Architectural aluminium follows QUALICOAT, and the 26th edition sets no stage count. It sets process limits, and the stage count follows from meeting them with the chosen chemistry.
The 26th edition requires an etch stage that removes at least 1.0 g/m² of metal, using one of the permitted etch types A1, A2, AA1 or AA2. It requires a conversion stage, and a final rinse with deionised water at no more than 30 µS/cm. On new lines it must be possible to take a sample after every stage, which puts sampling points in the tunnel design. The detailed limits, the 2.0 g/m² etch for SEASIDE work and the dryer and storage rules are in our post on what a QUALICOAT-class aluminium line needs from the equipment.
The comparison below sets the four process families side by side. "Not reviewed" marks the points where Annex 3 of QUALISTEELCOAT decides.
| Process | Stages | Bath temperature | Process window | Ceiling or rule |
|---|---|---|---|---|
| Iron phosphate | One as a cleaner-coater, more with separate degrease and rinses | 40 to 60 C | 2 to 3 min spray at 1 to 2 bar (M-FE 3803) | C3 under QUALISTEELCOAT |
| Zinc phosphate | Multiple, with conditioning before phosphating; produces sludge | 55 C or above | Supplier process sheet | Annex 3, not reviewed |
| Zirconium or silane thin-film | One supplier states at least five; sensitive to contamination | 20 to 40 C (M-NT 1) | 20 to 120 s spray at 0.5 to 1.5 bar (M-NT 1) | Annex 3, not reviewed |
| Architectural aluminium | Etch, conversion, deionised final rinse | Supplier process sheet | Etch at least 1.0 g/m², final rinse 30 µS/cm or less | QUALICOAT 26th edition |
Stage length at 1.5 m/min follows from the data sheet contact time
The length of a spray stage equals the line speed multiplied by the contact time on the chemical data sheet. We take the documented complete plant at 1.5 m/min and the two Henkel process windows from the table above.
Line speed = 1.5 m/min Spray iron phosphate contact time (M-FE 3803) = 2 to 3 min Iron phosphate stage length = 1.5 x 2 = 3.0 m, up to 1.5 x 3 = 4.5 m Zirconium contact time (M-NT 1) = 20 to 120 s, which is 0.33 to 2.0 min Zirconium stage length = 1.5 x 0.33 = 0.5 m, up to 1.5 x 2.0 = 3.0 m
The figures are active spray length only; drain zones between stages come on top. The calculation also assumes the conveyor holds 1.5 m/min through the tunnel; a slower line shortens every stage in proportion.
The documented complete plant has a two-bath spray pretreatment. Under QUALISTEELCOAT v5.0.1 that is an iron-phosphate-class process, and parts from it are capped at C3.
A fabricator brings a drawing for outdoor steel frames marked C4 and plans to run them through an existing iron phosphate tunnel. Under QUALISTEELCOAT v5.0.1 that chemistry stops at C3. The choice on the table is between accepting C3 with the customer or adding the stages a layer-forming conversion coat needs. That choice is settled with the chemical supplier's process sheet before any tunnel length is drawn.
A job coater takes in hot-dip galvanised parts alongside black steel and blasts them early in the shift, then queues them behind steel work. The three-hour limit after sweep blasting runs out while the parts wait. The fix sits in the layout and the shift plan: blasting close to the pretreatment entry and galvanised batches run as a block.
The pretreatment tunnel calculator sizes the stages at your line speed
Our pretreatment tunnel sizing calculator takes the metal, the duty of the coated part, spray or dip, the line speed and the contact times. It returns a stage sequence, the length of each stage, the tank volumes, the dryer and a flow diagram. Replace its default contact times with the windows on your chemistry's data sheet, and treat the proposed sequence as a starting point for the talk with your chemical supplier.
What this does not tell you
The worked example covers spray length only. It leaves out drain zones, heating capacity per stage, pump flow, nozzle layout, water treatment and effluent. The QUALISTEELCOAT copy we read sits on an Italian licensee's site and should be checked against qualisteelcoat.net before a quote relies on it. Annex 3, the matrix of systems by class, was not extracted, so this post does not say which thin-film systems qualify for C4 or C5. The stage counts for zinc phosphate and zirconium come from suppliers and can differ between processes.
Standards and sources
- QUALISTEELCOAT v5.0.1 with annexes, valid from 1 January 2026: C3 cap for iron phosphate, C2 limit for one-layer powder, salt spray hours by class, 3 h limit after sweep blasting, 24 h and 8 h pretreatment-to-coating limits, section 3.6.1 oven checks.
- QUALICOAT 26th edition, Volume 1, valid from 1 January 2026: etch of at least 1.0 g/m², etch types A1, A2, AA1 and AA2, sampling after every stage on new lines, final rinse of 30 µS/cm or less.
- ISO 9223 and ISO 12944-2: corrosivity categories C1 to C5.
- EN 13438: powder organic coatings on galvanised steel.
- ISO 9227 and ISO 6270-2: test methods named with the QUALISTEELCOAT classes.
- Henkel technical data sheets: BONDERITE M-FE 3990-1 (October 2019), M-FE 3803 (August 2020), M-NT 1 (March 2025).
- Supplier sources: DuBois white paper on pretreatment (2018), Chemetall Oxsilan product information. Products Finishing on low-temperature zinc phosphate.
Questions buyers ask
How many pretreatment stages does a powder coating line need for steel?
A powder coating line for steel needs as many stages as its chemistry and target class require. Iron phosphate can run as one combined cleaner-coater stage and serves up to C3 under QUALISTEELCOAT v5.0.1. C4 and C5 work needs a layer-forming conversion coat, and zinc phosphate or thin-film processes run in multiple stages.
Is iron phosphate enough for outdoor steel parts?
Iron phosphate is enough up to corrosivity category C3 under QUALISTEELCOAT v5.0.1, which states that C3 is the maximum for pretreatment based on iron phosphate. A one-layer powder system on steel is limited to C2. Parts specified for C4 or C5 need a different conversion chemistry and a longer pretreatment sequence.
How long can parts wait between pretreatment and powder coating?
Parts can wait up to 24 h for classes C1 to C3 and up to 8 h for C4 and C5, according to QUALISTEELCOAT v5.0.1. Hot-dip galvanised parts must be coated within 3 h of sweep blasting. For aluminium, the QUALICOAT 26th edition limits storage of pretreated material to 16 h.
Does zirconium pretreatment need heated tanks?
Zirconium thin-film pretreatment runs at low bath temperatures. Henkel's BONDERITE M-NT 1 data sheet gives 20 to 40 C with a spray time of 20 to 120 s at 0.5 to 1.5 bar. Iron phosphate, by comparison, runs at 40 to 60 C and zinc phosphate at 55 C or above, which makes zirconium the coldest of the three conversion processes.
Bring the corrosivity class from your drawing and your chemical supplier's data sheet when you plan a line; the class and the contact times fix the stage sequence and the tunnel length. Our pretreatment lines cover spray washing lines and dip pretreatment tanks, and the contact form takes the drawing and the data sheet for a proposal.