Question Discoloration after ultrasonic bath (Apeks first stage)

[awesome-diver]

Hello
After an ultrasonic bath for 15 minutes with a temperature of 38 degrees the brass parts, valve lifter and spring adjuster, are discolored with black spots. My cleaning solution contains a drop of dishwater detergent and a tiny bit of vinegar concentrate. It seems just to be an optical problem however is it possible to prevent this next time and what are possible causes?
Thank You

 

[awesome-diver]

My current hypothesis is that these brass parts have some kind of coating and the ultrasonic cleaner removed some of the coating. Any ideas?

 

[admikar]

There is no coating on these parts.
I never use concentrated vinegar, only 50:50 water/consumable vinegar

 

[awesome-diver]

The Vinegar concentrate I use is for consumption as well, but you have to add 4 times the amount in water first, to get a 5% Vinegar. My solution is much weaker then the 50:50 water/consumable vinegar solution. When its not the solution and there is no coating, maybe 15 minutes is just too long for these two parts? Did anyone experience something similar?

 

[greeniguana]

The longest I've ever ran a reg part in ultrasonic vinegar is 5 min.

 

[Fibonacci]

Could be that the electroless nickel plating was originally passivated to improve corrosion resistance.
I normally never use the ultrasonic cleaner with an acidic bath (either 50:50 white vinegar or citric acid) just Simple Green Crystal 1:20.

There was an interesting thread here:
Cold Water Passivation of Nickel Plating
To answer your question, one should first know what passivation is. Passivation is a process or the phenomenon by which a metal surface remains un-attacked by corrosion because the metal surface conditions have been protected.
Like Manganese, titanium, Aluminum or steel, Nickel has a strong affinity for oxygen in air or water.
Therefore in cold water Nickel can form a stable oxide film on its clean surfaces to become passivated. The formation of passive oxide film on its surface is accelerated when rinsed in clean hot water below a boiling point. In passivation (hard oxide growth film) process, nickel is made the anode and its passivation improved by rinsing it with either a dilute solution of nitric acid, chromic acid, citric or sulfuric acid; for example and then thoroughly rinsing with deionized water.
The nitric or chromic acid oxidizes and dissolves any impurities on the surfaces of the Nickel.
The deionized water rinses off any remaining organic acid and inorganic impurities allowing the stable oxide film to remain on the nickel for corrosion protection. The part would then be blown dry, leaving the oxide film, strongly bonded on the nickel surface.
It is important to note metal that do not form strong affinity with Oxygen, may be successfully passivated.
The key point in obtaining a superior oxide growth film on nickel is proper surface pre cleaning to remove oil, dirt and other metal impurities.
It would be a mistake to finally rinse a passivated component such as nickel from boiling and or strong acid of low pH water since such would etch off or reduce the thickness of the oxide growth film on the metal, rendering it prone to less corrosion protection...

 

[flymolo]

15 minutes seems like a little too long. I usually ultrasonic my regulator parts for 5 minutes max