#Pitting patina aluminum cracked#

One of the two aluminum parts has cavities for cooling and all the internal faces which are open to the liquid show pitting! This is the biggest indication that the liquid itself is causing corrosion. I do not yet know what this is but it could be ethylene glycol. There is also some type of anti-corrosion coolant added to protect the copper pipes. It is city tap water but there are copper ions because 90% of the piping is copper.

In addition, there would be leakage current while operating at continuous current.ģ) Very corrosive cooling fluid - which is probably the main culprit. The temperature cycle was from 25C - 70C so crystallization of the EN is not likely, however, there are other factors:ġ) Internal pressure - the two parts are under bending stress when the system is operating.Ģ) There are large electrical fields that would create induced currents throughout the material during operation under a pulsed format. Interestingly, when some of the less corroded surfaces are wiped off, there is a green residue on the cotton. Both the nickel and gold are used only for corrosion resistance. The plater is certain that the parts were not heat treated. It is as if the water could not permeate the localized area around the bolt hole. What’s funny is that the areas of least corrosion are around bolt-hole locations. You may be correct that the plating is suspect. We are using one of the few platers in our area and unfortunately they do not conduct monthly salt-spray qualification tests. Thank you for such an excellent and extensive reply! This is a complicated issue because there are many things happening at once. RE: Plating corrosion on gold over nickel plated 6061-T6 aluminum THA88 (Aerospace) Gold can cause galvanic corrosion of Ni (green hydroxide or blackish hydrated oxide), but in this case, the Al sacrificially protects the Ni.

I don't think it played a significant role. Was the gold added for corrosion resistance or for electrical purposes? Probably too thin to fix any EN problems. What is the cooling fluid? Maybe switch to auto antifreeze/coolant. Any indication corrosion started at the edge of the unplated holes? Can tell by microscopy & x-ray diffraction, or from salt spray test after thermal cycling.ĭescribed the bolting.

#Pitting patina aluminum cracked#

Perhaps the temperature caused EN crystallization from the amorphous state, which cracked the thick EN. What is the temperature cycle? Low-, medium- or high-phos EN? The nickel coatings shall be of the following grades: Although Class 1 specifies no HT, maybe done accidentally or via your heat cycle.Īre you sure about your EN Grade B thickness of 0.002-.0025 inch? Per MIL-C-26074E,ġ.2.2 Grades.

Heat treatment of the EN improves adhesion but worsens corrosion resistance. Usually only a problem with low-phosphorus EN, mid-phos normally passes & high-phos passes with flying colors. The salt spray required per MIL-C-26074 is 100 hours. Cu (& Cl -) ions in solution are also bad for aluminum.Īsk for a copy of the plater's monthly qualification results for salt spray (per ASTM B117) & adhesion testing (180 o bend). The corrosion can be quite rapid if the plating adhesion is poor. Nickel is only a barrier coat, not sacrificial (like Zn on Fe). Galvanic corrosion of aluminum (white hydroxide) from nickel plating is common if porous or cracked plating.