Can Aqueous Alkaline Cleaner Be Used on Aluminum?

Q. Should I not use the same bath to process aluminum and stainless steel? Can aqueous alkaline cleaner be used on aluminum?


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Q. We use an aqueous alkaline cleaner to process parts with ultrasonics and filtered circulation. The process does well on stainless steel parts, but I have an issue with aluminum parts, using the same parameters, minus the ultrasonics. The aluminum parts come out of the bath with dark, black streaks and spots. Should I not use the same bath to process aluminum and stainless steel? Can aqueous alkaline cleaner be used on aluminum?

A. Probably not. Cleaners that are formulated to clean metal like stainless steel usually cannot be used for aluminum. Since industrial processing of stainless steel (forming, bending, cutting, etc.) is typically more demanding than a similar operation for aluminum, the lubricants used can be difficult to remove. Therefore, most aqueous alkaline cleaners formulated for use on stainless steel have a high pH and will contain quite a bit of free caustic in the form of sodium or potassium hydroxide (in addition to ingredients like surfactants and buffers).

If a cleaner like this is used on aluminum, it tends to etch the surface. This condition will be made worse with elevated temperature and longer periods of time. Additionally, the higher the concentration of the cleaner, the worse the etching and discoloration.

Free caustic will quickly reduce the naturally-occurring oxide on aluminum and immediately start to etch and dissolve it. Most industrial aluminum is alloyed to attain specific properties (typically with copper, manganese, silicon, magnesium and zinc). The discoloration of the aluminum is the presence of the undissolved alloying elements and the secondary phases that they form that are not attacked in the high pH bath. They wipe off fairly easily, although chemical removal is generally only attained by use of an acid (like nitric) to dissolve these alloying elements. 


How to Improve Zinc Phosphate Quality

Q. I am a quality inspector and want to know some of the reasons for poor phosphating. Our customer complains that they saw a white spot in the zinc phosphate coating. How do I troubleshoot my zinc phosphate quality issues?

A. The zinc phosphate chemistry and process is complex and there are several potential failures. If the problem appears to be one that is localized to a few parts, I suggest further examination of your racking and rinsing process. 

It is important to identify problem parts as soon as they come off the line. Are the spotted areas occurring where the parts contact the rack? Are they occurring in an area with the most draining of fluid from the parts? If so, you may need to consider a new method to rack the parts, an alternative material for the rack itself or make improvements to the rinse water quality. This may involve more frequent dumping of the tank or an increased overflow. A good way to monitor and maintain consistent water quality is to use a conductivity sensor hooked directly to a solenoid valve that will open when the conductivity of the water becomes too high.


How to Control Citric Acid Concentration in Wastewater Treatment

Q. We are treating ammonia wastewater, however, it is connected with the sanitary wastewater, which is often blocked. We have used citric acid to effectively clean it in the past. How can we tell the concentration of the acid and whether or not it will adversely affect the system?

A. It is easy and relatively quick to make up a control procedure for a simple acid or alkaline solution. In your case, you should make up a solution of citric acid at approximately the same concentration that you used by adding a known weight of citric acid to clean water (ideally deionized or distilled).
You then accurately sample it (25 mL, for example) and titrate that solution with a base of known concentration (0.1 N sodium hydroxide, NaOH) using phenolphthalein as an indicator. As you add more drops of the sodium hydroxide standard to the citric acid solution with stirring or agitation, the phenolphthalein indicator will turn from colorless to pink. At that point, the titration is complete. 

The next step would be to determine a strength factor for any future titrations by dividing the concentration of the citric acid solution you originally made by the number of milliliters of the sodium hydroxide standard solution used to get to the equivalence point of the titration (when the color change occurred).

Now that you have established that relationship by developing the strength factor, you can titrate any future citric acid solution of unknown concentration by repeating the sampling and titration procedure. You will need to sample the same quantity as you did to establish this procedure for the strength factor to be accurate. Since citric acid is a relatively weak acid, it would not be expected to be harmful to the materials of construction of most standard wastewater treatment systems.


How to Select the Right Washer Line Material

Q. My company plans to install a six-stage washer that includes an alkaline cleaning tank that will operate at about 130°F and an acid deoxidizing stage that will operate at about 90°F. Would you recommend polypropylene or 316 stainless steel for the tank material?

A. The choice of washer material will be based primarily on the durability requirements you will need along with the intended service life of this cleaning system. An open, non-pressurized tank is compatible with the temperature range you want it to work in. I would expect the polypropylene to be good up to about 160°F. It is also compatible with almost all standard chemicals used in metal finishing except for chromic acid. The 316 stainless steel would be equal in compatibility with these chemicals, but would be much more expensive. The 316 stainless steel grade would be the preferred stainless steel, having more corrosion resistance than the 304 grade. 

Depending on the size and weight of your parts, the stainless steel construction would be much more durable. I have seen plastic industrial piping hold up well to the temperature and chemistry of most metal finishing solutions and operations, but then get destroyed by accidents and maintenance activities.



Originally published in the March 2017 issue.