Anodizing: Corrosion on Hardcoat Anodized 7000 Series Forging


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Corrosion on Hardcoat Anodized 7000 Series Forging 

Q. I sent you photos of two parts that are alloy 7050 forgings. On these parts there are spots that appeared as a result of Type III (hardcoat) anodizing that were not present before anodizing. They look like corrosion of some sort, but the corrosion looks a little different on each part. What causes these spots to appear and what measures can be taken to prevent or eliminate them after they occur?

A. I can see from the photos what appears to be electrolytic pitting on one part and what’s often called “snowflake corrosion” on the other. There are a number of conditions that can promote corrosion of this type. The key to discovering the cause and its remedy is for the anodizer to carefully observe the parts at each step in the process to determine which tank the corrosion occurs. Then they can do whatever it takes to try to prevent it.

This alloy is very active because of its relatively high amount of zinc. The Aluminum Association alloy designation tables show zinc in the range of 5.7 to 6.7 percent. Zinc is second only to magnesium of the most active metals on the chart of galvanic activity. The 7000-series aluminum alloys follow close behind zinc. This means that 7050 alloy can be very sensitive to electrolysis, also called electrolytic corrosion. 

The questions to ask are have these parts been run successfully in the past and is this the first time this condition has been seen? The most common occurrence of electrolytic corrosion is caused by stray electrical currents that surge throughout the plant from time to time. There are two ways stray currents can get to the parts, one is through the overhead crane; the other is directly through the tanks, if they are metal. 

There are ways to prevent this from happening. If the tanks are metal, make sure they are properly grounded. The best way to do this is to weld all of the tanks together with metal straps. Stainless steel angle make for a good strap, then weld a strap from any tank to the closest metal water line. Test all the tanks with a meter for proper ground, and if there is no metal water line available, find a nearby solid ground such as an anchor bolt of a steel building column. If this is not possible, a method that is also effective is to drill a hole through the concrete floor (not in the immediate vicinity of the tank line) and insert a long solid copper rod or bar well into the ground. Connect the ground to one of the straps or to a tank with a solid connection, then seal up the hole around the copper ground with silicone caulk or other suitable material. Non-metallic tanks do not need to be grounded.

All of the cranes over the line should be electrically isolated from the loads. Usually, the easiest way to do this is to insulate either the pick-up points on the anodizing load bars, or insulate the pick-up points on the hoist. Every hoist/crane/load bar pick-up design is a little different so you have to come up with a solution that will work for your situation. Whatever is done, make it rugged and permanent. You don’t want to be replacing the insulation frequently because it wears out quickly. Some fixes will be more difficult than others, but it is worth the time and effort to get it right, no matter what.

If parts are left in a rinse tank too long, they can develop electrolytic corrosion in spite of best efforts to ground and isolate. With 7050 alloy, avoid letting the parts sit in any rinse tank at all. They should be rinsed and moved immediately to the next process step as a precaution. Make sure loads are electrically insulated from metal tanks by using a suitable plastic plate at each end of the tank to rest the loads on as they progress through the line. Polypropylene is a cost-effective option for this. Other materials include PVC or a glass-epoxy phenolic called G-10. Some anodizing lines use oak boards or 2 x 4s. These are effective, but they can eventually become saturated with water or chemicals, which causes them to lose their effectiveness as a dielectric.

It is safest to use aluminum racks for these parts in order to prevent any possible galvanic action between dissimilar metals, i.e., the aluminum parts and titanium racks. This can sometimes happen when the surface area of the titanium rack is larger than the surface area of the aluminum parts. It’s something to be aware of.

Electrolytic corrosion seems to happen most often in the aqueous cleaning tank. Nevertheless, it can happen in any tank. I have never known it to happen in the anodize tank.

Your anodizer is probably aware of all of these things. However, even if the tanks are properly grounded and hoists are electrically insolated, it does not mean that this is a permanent fix. It is best to make this part of a regular preventive maintnance check, perhaps on a monthly basis.


Keeping the Lab Records

Q. What type of software would you recommend to track the chemistry on an anodizing line? We currently use Microsoft Excel spreadsheets.

A. There are various software products available that will do this. An internet search will bring up a few canned programs. Creating your own custom forms using Excel is also a simple and effective way to manage the anodizing bath parameters. That way the forms can be developed to exactly fit your own system. I have used this method in the past using “X bar R” (high and low limits–simplified Six Sigma-type charts) to transfer the titration, pH or even temperature results onto. The charts can be particularly effective if copies are physically posted in the anodizing department each day for all to see. It only takes a quick glance at the charts to see whether a process is under control or not. 


Welded Aluminum Railing Discoloration

Q. We fabricate railings that are primarily steel, but sometimes aluminum, using 6061 alloys or occasionally 6063. Because this is structural railing, the welds have to be strong enough to meet the specs. Nearly all of the aluminum rail is sent out for anodizing. After anodizing, some of the parts show a dark area or a multi-colored halo around the welds. The discoloration ranges from gray to brown to black. Sometimes some of it can be wiped off, but the weld area is still discolored. We suspect this condition is either a welding issue or an anodizing problem, or perhaps a little of both. Our anodizer says it’s a welding problem. 

A. The anodizer is correct. This is not an anodizing problem; it is a welding problem. Virtually all aluminum railing fabricators have had to deal with it at one time or another. It’s largely a matter of perfecting a welding technique that works. The alloy of the welding wire is also important.

One of the most common welding wire alloys for aluminum is 4043. This alloy should only be used if the parts will not be anodized. The high silicon in 4043 makes it easy to weld and gives a strong joint. However, silicon does not anodize and is not homogenous with aluminum. When 4043 wire is used to weld parts that will be anodized, the result is a black, powdery weld that is not acceptable, from an appearance standpoint. The most common alloy filler wire that is used for “architectural” applications that must meet certain structural standards is 5356. Alloy 5154 welding wire can also be used for parts that will be anodized.

The goal is to produce the weld strength required, but without getting the welded railing members too hot during welding. When the welding temperature is too high, the chemistry of the extruded aluminum members is affected and the alloying elements, namely silicon, magnesium and chrome, come out of solid solution and precipitate on, or near, the surface of the extrusion. This is what causes the various halo color effects. The discoloration is usually not visible until after the railings are anodized. 

One way to solve this problem is to set up a test using different welding techniques to arrive at the correct formula. The correct formula will achieve adequate structural strength to meet the specs without adversely affecting the appearance. Keep track of exactly what is done to each sample and mark the samples so they can be identified after having them all anodized in the same batch. Select the best combination of strength and appearance.