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We have an anodizing line and we have been battling “water marks” under the seal. This has been more prevalent on flat parts and clear anodize, it is also present when we color (2-step color or electrocoloring) parts dark.
Through the process, we do not notice any beading of water, through each step we have a consistent water break test. Our rinse tanks after each process is overflowed and held to a pH between 5.5 – 7. Our seal is a magnesium based seal and we just measured the seal and it is within the allowable concentration % range. We do run agitation on the seal
After the seal we have a hot rinse and the pH was measured to be around 6.5, we continually overflow that as well, to keep clean water in the tanks. We do have agitation on the hot rinse and the temperature of the hot rinse is ~145F.
We are currently chasing our tail.
Thanks,
Kyle
Kyle, the detail that stands out is that your water break test is consistently good through every step, yet the marks appear under the seal. That combination is worth taking at face value, because it rules out the usual explanation and points somewhere more specific.
If the surface is genuinely water-break-free going into the seal, then the water you are seeing marks from is not the rinse water sitting on the part before sealing. It is water that arrives at, or is drawn into, the surface during and after the seal itself. A magnesium-based seal runs hot (typically 95-100C) and the part leaves it wet and hot, so the few seconds between the seal tank and the hot rinse are the window where marks are formed. On flat parts and clear anodize they show worst because there is no texture or colour to hide a faint dried deposit, and flat faces also drain as a film rather than shedding as droplets.
A few things I would check, roughly in order of how cheap they are to test:
1. Dwell and drainage between seal and rinse. Time how long the rack actually takes to get from the seal to the hot rinse, including the travel and any drip time over the tank. If a flat part is standing in air for 20-40 seconds while hot and wet, dissolved seal chemistry concentrates as the film evaporates and leaves a deposit. Reducing that transfer time, or adding a brief drag-out rinse immediately at the seal exit, often changes the pattern dramatically. If the marks move or reduce when you change only the transfer time, you have your answer.
2. Rack orientation and drainage paths. On flat parts, note which face is up and where the water collects against a rack contact or a masked edge. Marks that follow a consistent position relative to the rack rather than a consistent position on the part point to drainage and contact, not to chemistry.
3. The hot rinse itself. About 145F with agitation is reasonable, but if the marks are deposits from the seal drag-out then the hot rinse is re-dissolving and re-depositing them rather than removing them. Check whether the hot rinse is actually overflowing fast enough to keep the dissolved solids low, and whether its pH has drifted. A hot rinse that looks clean can still carry enough dissolved seal salts to leave a mark when it dries on a hot flat surface.
4. Drying. Marks “under the seal” are often described that way because they are visible after sealing, even when they were laid down during drying. If the parts are allowed to dry slowly in still air, any residual film on a flat face concentrates into a visible watermark. A controlled dry – or a final rinse that leaves nothing behind – is often what actually fixes this.
Two diagnostics I would run to separate these:
– Take a set of flat parts and pull them at each stage: after seal, after hot rinse, after dry. Photograph and compare under the same lighting. The stage where the pattern first appears is the stage to fix, and it is not always the one you would guess.
– Deliberately vary one variable at a time on a small rack: transfer time first, then hot rinse overflow rate, then drying method. Changing several at once will make it look like it is fixed and then it will come back.The one thing I would not do yet is change the seal concentration or the anodize chemistry. Your seal is in specification and your water break is good, so the evidence points at the wet-handling between the seal and the dry, not at the bath. Chasing the baths first is what usually turns this into weeks of tail-chasing.
Worth noting as background: this is the same class of problem as rinse water entrapment and staining in any wet line, and it is why we treat the transfer and dry steps as controlled process steps rather than as handling. In our own work on mechanical finishing the equivalent is trapped compound and media residue showing up after the part looks clean, and the fix is almost always in the rinse/dry sequence rather than in the chemistry.
I work for a manufacturer of vibratory and mass finishing equipment, so anodizing line chemistry is not my commercial area – I am answering from surface-condition and wet-handling experience.
Max Xu
Surface Finishing Engineer, SurfacePolishAre the parts still mounted on a rack, or arranged in the order they were on the rack, such that the problem is seen to be getting worse the lower down on the rack?
A waterbreak after an alkaline tank or alkaline tank is not convincing. Is there a chance that there is a very short etch but then a long pause for draining before getting to the etch rinse, such that these stains are actually etching?
Thanks for all the input, this gives us a place to start the Root Cause Analysis.
Ted from your questions, in the picture shown, the parts are still on the rack in the way that they were racked. If there are marks they are consistently in the same area from part to part on the rack, but the marks vary location from rack to rack. We have found that the marks are more visible on flat parts in clear anodized finish, but they are still visible, but less so on round parts and dark electrocolor finish.
Our current process is caustic etch for 5 minutes then a 15 second dwell time prior to going into the 1st etch rinse. From the seal to the hot rinse there is a 5 second dwell time.
If practical I’d love to see if there is a difference with a 5 second dwell after lifting out of the etch rather than 15 seconds.
Hey,
If the marks are truly under the seal, verify the seal itself before chasing handling. Run a dye-stain test per ASTM B680 on a marked area versus a clean one — if the seal is intact under the marks, you’ve ruled out seal chemistry and it’s purely a wet-handling/drainage issue. That one test can save weeks of tail-chasing. Our lab routine: https://www.yumeihardware.com/ Disclosure: I’m with Yumei Hardware.The observation I would chase hardest is this one: a consistent water break test, and still water marks. In our experience those two facts together point at the failure mode more than anything else on the line.
A water break test detects hydrophobic films, not dissolved solids. It tells you there is no oil or grease film, which is valuable and you have clearly controlled. It tells you nothing about what is dissolved in the water left on the part. A water mark is very often the evaporated residue of exactly that film – the dissolved solids it carried, concentrated as the water left. So the test you are using to prove the surface is clean cannot detect the thing that is marking it. That would explain chasing the tail.
A cheap way to confirm it is a control part: keep everything the same except give one load a deionised final rinse and forced-air drying immediately after. If the marks go, you are looking at evaporite and water handling, not at seal chemistry.
Three things follow from that, and all are cheaper than changing the seal:
- Measure conductivity of the final rinse, not just pH. pH 5.5 to 7 is reasonable and overflow keeps the contamination load down, but neither number says anything about total dissolved solids. Conductivity climbing through the day is the variable to watch, and it is the one that tracks with marks appearing later in the shift.
- Treat drying as a process step with an exit criterion. Parts that sit wet, and especially parts that sit wet in contact with each other, dry unevenly and mark at the contact and pooling points. Forced air with a defined dwell is different in kind from letting them stand. In our own work we design drying and media separation as a stage for this reason rather than leaving it to time.
- Flat parts are the tell. A horizontal surface holds a film that evaporates in place; a vertical or well-drained part lets the film run off before it deposits. That your worst cases are flat and clear anodize fits evaporative deposition better than it fits contamination, because clear anodize shows the deposit while colour hides it.
One more thing worth checking, since it is easy to overlook: the hot rinse after the seal. Magnesium-based seals leave salts behind, and the hot rinse is what is supposed to take them away. If its temperature falls as cold parts load it, its ability to flash the part dry falls too, and the residue that was going to leave instead stays and concentrates. Log the temperature at the start and end of a load rather than the set point.
I will keep to what I can speak to honestly: we build mass-finishing equipment and media, not anodising chemistry, so I am not going to advise changing the seal or its concentration. What I am confident about is that the water break test is the wrong instrument for this failure, and the control part above will tell you quickly whether that is where the tail is.
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