Pitting on 7000 Series Al Alloy
In the last year, we’ve had a string of anodized parts come back from the finisher that are “pitted.”
Q. We manufacture and sell a line of high-end bicycle components. The parts are machined from an extruded 7000 Series aluminum alloy and then are bright-dip anodized either clear or dyed black. The parts are then engraved with a CO2 laser with our logo, the part number and other identifying information.
In the last year, we’ve had a string of anodized parts come back from the finisher that are “pitted.” The parts showed no signs of pitting prior to anodize, so we initially pointed the finger at the finishing house. After a visit to the anodizer, however, we are beginning to think it’s something that we are doing in-house to cause the problem prior to finishing. Whatever we’re doing to cause this pitting condition isn’t apparent. The only pattern is that the parts that tend to pit first get turned in a CNC lathe. From there, they are machined in a horizontal, four-axis milling center. The surfaces that pit are the same ones that are turned on the lathe and that are clamped by hardened steel fixtures as the finish machining is completed on the mill.
We thought it might be a coolant concentration problem, but the machines get regular cutting fluid changes and, when tested, the pH levels were not out of the ordinary. One other possible cause that we haven’t ruled out is a bad ground in the machine. Could there be small levels of voltage jumping through the tombstone and steel fixtures to the parts while they’re being held for machining? And if there was a traceable voltage, could that cause the pitting?
Our part-cleaning system consists of an operator dipping the machined parts in semi-hot water to rinse off the coolant. Please steer us in the right direction. I hate seeing beautifully machined parts thrown away. —E.L.
A. You seem to be aware of some of the conditions that could possibly affect the surface condition of the parts.
It doesn’t appear to me that anything in the machining process is causing the pitting condition you are concerned about. I assume the machining coolant is water-based, and you say the parts are being thoroughly rinsed with warm water after machining. You have not said how the parts are dried after they are rinsed, however. It is possible that if the parts remain wet, the water could start some corrosion which, in turn, could cause some light pitting, virtually undetectable. The high level of zinc in 7075 could exacerbate the corrosive action since zinc is a very “active” metal. However, if these parts are packed wet in such a way that they are not allowed to dry, they could easily start to corrode in as little as 24 hours, maybe less. If the parts are corroded by the time they are introduced to the anodizing process, the bright dip could exacerbate the condition.
Alloy 7075 can react to the anodizing process in a unique way that other aluminum alloys do not. Sometimes the parts will exhibit blistering, which is initiated during the anodizing process. The following is part of an answer I wrote in this column in the November 2011 issue of Products Finishing, pertaining to blistering on 7075:
“As I have learned more about the metallurgy of aluminum and what actually takes place as the alloying elements try to diffuse through the anodic coating as anodizing is taking place, I believe that finally we have a viable explanation of how and why blistering can sometimes occur on high zinc alloys. Dr. Jude Runge writes:
‘Blistering in 7000 series alloys is often mistaken for hydrogen blistering. Aluminum is not susceptible to hydrogen embrittlement, except in extremely rare cases! Blistering occurs a lot more frequently and this is why: The diffusion model of the anodic process shows that only aluminum is anodized, and the short-range structure that grows and develops into the AAO (anodic aluminum oxide) is composed only of aluminum and oxygen. This leaves the other alloying elements behind, either entrained at the interface, or jumping through the molecular holes in the anodic coating semiconductor structure into the electrolyte. Zinc, especially when it oxidizes, is too big to jump through the holes in the structure. It remains at the interface between the substrate and the AAO, and it oxidizes under the anodizing conditions as it sits at the interface. This creates a loosely adherent boundary between the anodic oxide and the aluminum substrate. It results in a naturally occurring stress point between the finish and the substrate, so now we have moist “gravel” in the form of zinc oxide at the interface. Any subsequent thermal cycling may cause these areas to delaminate or blister. It can be that the blisters are simply not visible until after dyeing, that the process parameters of dyeing allow them to really develop. If a longer initial ramp period is used to get that zinc moving away from the interface early in the anodizing cycle, before the finish gets too thick, the occurrence of the phenomenon goes way down…’ ”
“So we can now understand that the root cause is the high amount of zinc in the 7075 alloy. The degree of blistering, or whether it even occurs, can certainly vary from batch to batch of metal. Since the zinc may not be perfectly evenly distributed throughout the metal substrate, some machined 7075 parts may have blistering and some may not. Some parts may have blistering on one surface but not another, such as in your case. Because the alloying elements, to put it simply, migrate through the aluminum-AAO interface at a different rate than the aluminum does, it appears that the best chance of reducing the occurrence of blistering is to use a very slow ramp rate (longer ramping period) when the current is first turned on. The faster the current is ramped up, the faster the aluminum migrates from the substrate to the anodic surface and the faster an anodic coating is built up. This can increase the chances of the alloying elements, especially zinc, being ‘left behind,’ or trapped, at the interface. This causes that loosely attached anodic coating and increases the opportunities for subsequent delamination of the coating, due perhaps to the stresses of thermal cycling caused in the dye or even the seal.
“Try using a ramp period of at least 10 minutes and see what happens. If this reduces the occurrence of blistering, but does not completely eliminate it, try a 15 minute ramp to full current density and see if that helps.”
The blistering could happen in the anodizing tank, dye tank or seal tank. In the case of your bicycle parts, when a “blister” forms on the surface it is relatively easy for the anodic coating at that point to pop off of the substrate. What you see as a result is a pit. I can’t say for certain if this is what is happening, but it is one plausible explanation. If this continues to be a problem, you might want to consider using a different alloy such as 2024 or even 6061.
In this paper, a review of several process solutions, examining coolants, solvent cleaning, alkaline clean/etch and deoxidizing/desmutting, listing intended and unintended chemical reactions along with possible mechanisms that would favor corrosion formation.
The following anodizing process overviews are provided as a means of introduction to aerospace anodizing
This important first step can help prepare the metal for subsequent surface finishing.