It definitely is NOT zinc Rot, it is however very likely that it's Powdery Rot. This has already reared its head in both 1:200 and 1:400 scales of late and discussed at length.
Recent discussion of Powdery Rot in 1:400 scale on DAC
Recent Discussion of powdery rot in 1:200 scale on DAC
Originally written by mach25 in the thread referenced above explaining how powdery rot damages models.
I thought I would "perhaps" shed some light in the explanation of the potential issues of metal degradation or "rot" as commonly called.
I am the person in Japan that asked my brother who is a Chemical Engineer at Dupont Chemicals in the United States, regarding the tarnishing in the 1/200 polished metal models a few months back and posted a thread on this issue. Since I was in town visiting him on a flight layover from Japan, I showed him this hotly contested thread during dinner and asked him for his input. I'm just a pilot but he's an expert on this issue.
Here's his very basic input and thoughts, based on not having the Gemini model for testing at the laboratory.
As the moderator make.me.laugh very correctly stated, aluminum and all metal for that matter, if not treated correctly, will turn back to it's original ore state at one time or another. The term "rot" commonly used, such as Zinc Rot etc., but in actuality, the metal is not rotting but due to peripheral circumstances (air, water, chemicals (organic and inorganic), heat, cold etc.) is reverting back to it's very basic original ore state. Whether is cast silver aluminum or aluminum grey-white power, it's still basically aluminum as an element. It's the Mass Conservation Theory of "Matter can neither be created or destroyed". Although it may change form, it cannot be destroyed. But a simple name rather than degradation and other scientific mumble-jumble name is simply "rot".
Degradation process can begin immediately and take days, weeks, months or years before notice. It really depends on the state of the original metal, how it was processed and the environmental factors, past and current.
For the most part, Chinese metal ore mining manufacturers and their metal distributers (where the company buys the metal from to cast their parts) are extremely "unregulated" and do not adhere to the ISO9000 series (International Standards Organization) of Standardization of Quality Control, Assurance and Management. Hence metal purity and quality as a general rule can become anyone's best guess. It's basically cost versus quality, as ISO adherence is very expensive and requires time and support from the company and employees.
For example, one of the concerns at Boeing who is building their aircraft factory in China, is where they and their sub-contractors are to source the metal vendors for their aircraft. The Chinese government is pressuring Boeing to source their metal vendors from China, yet, many of these vendors are not ISO9000 compliant and even if they are ISO compliant, are subject to shortcuts, "look the other way", "monetary compensation - bribes" for cost control, as per their 2013/2014 audit. Dupont Chemicals works closely with Boeing and other aircraft manufacturers and an entire division at Dupont Chemicals, is dedicated to working with them and during the audit, found the Chinese metal processing and quality assurance appalling.
In a very lowest common denominator, it's like comparing Reynolds Aluminum Foil versus a XYZ Discount Store unknown manufacturer brand of aluminum foil. The Reynolds may be 2 - 3 times more expensive, but you know what you're getting (for the most part). The discount store aluminum foil may break apart, not sustain heat and even be recalled by the Food Safety Department for having mercury inside of the foil sheets (yes...... this actually has happened in Asia).
When my brother did the destructive testing on the Inflight and Aviation 200 models etc., the aluminum body and wings contained multiple metals (some were trace elements) such as Iron, Tin, Nickel, Copper etc along with other human organic contaminants. The aluminum was basically not high-grade (pure would be impractical). Though he did not test for the exact type of aluminum alloy (there are multiple grades), the aluminum used was highly machinable but very low grade, perhaps due to this, is susceptible to corrosion and oxidation (from both external and internal reaction sources). Again, it's about cost, ease and time.
Imagine if you will, an aluminum soda can left out in the elements over time, it'll probably get brittle, as the aluminum is breaking down. Now compare that to the aluminum used on space crafts, the degradation process is much slower due to the grade of aluminum and how the aluminum was chemically treated. But still, the aluminum used on the space craft is degrading, perhaps a million times slower than the aluminum soda can.
Again, even the best surface treated aluminum can corrode, should the base aluminum contain contaminants or other metals within it. If not about "if" but how fast or how slow.
So generally speaking, where does the other contaminants and metal get into the aluminum casting? My brother explained that the aluminum ingots may already have other metals and contaminants inside of it. How it was mined from ore and the quality control measures required to keep it in a highest quality state, from the ore mine to the smelting pot to cast the ingots, is anyone's guess.
Should the manufacturer suspect or have suspicion that the ingots be already contaminants, there is a process to remove it, but again, it's expensive and time consuming.
At the manufacturer's site, when casting the parts, contaminants within the smelting pot if not completely cleaned out can also be the culprit. For many companies, they may smelter aluminum one day, iron the following day and copper the next day etc. To clean out the smelter pots is time consuming, expensive and difficult, it's not as simple as heat up the pot, pour out the remainder metal and wipe clean. To treat the pot correctly, it requires chemical treatment and cleaning the pot "without" damaging the pot, which may bring the actual pot material into the next mix. Hence, ISO9000 series manufacturers will never use the same smelter pot for different types of metals.
So why and what happens during the metal degradation process? Anything really can cause degradation of the metal (natural or induced), from air, chemicals and other contaminants, organic and inorganic. As my brother and perhaps had the best analogy..... "Metal degradation is like bread yeast". In order to make the entire bread batch rise, it only takes a tiny amount of yeast. The same holds true for metal degradation. It only takes a small amount of contaminants to spread like wild fire. Consider the metal contaminants like yeast, as it begins to degrade, it multiplies exponentially. In the metal model, very contaminant is finding a nook and cranny to spread and degrade. Now add oxygen or other chemical elements and you've just accelerated to process.
Even having a tiny pin-prick of a hole in the paint or area where it is not treated correctly, or just happens to be a trace element of a contaminant metal or another contaminant in the aluminum will lead to degradation process. It really only takes one oxygen molecule or another environmental factor to start the entire degradation process. In the case of the Delta 757, the process could have started from where the bare metal pylon attaches to the wing.... just as an example.
Why do you see cracking, warping and bubbling? Without again going into his scientific mumble-jumble, anytime something is changing from one form to another requires "energy". In these model, you may not be able to feel it, see it or smell it, but energy is being produced until the process is over (powered state in this particular case). When you're boiling water and see the bubbles forming, it's energy being produced and the element is changing form from a liquid to a gas. Essentially as the model parts degrades to it's basic ore state (powder), the same process is happening with released energy. If it's under dried paint (and flexible to a point), this energy has no where to go except forming bubbles (cracking the paint if enough energy is produced and the paint was not flexible) or if enough energy is produced, can actually warp out the part without breaking it (though a light tap will fracture the part as only the surface tension is holding the part together and the inside is completely porous and essentially "cooked off").
Can you mitigate the model from the degradation process? Unfortunately no. Once the process has begun, you can't stop it unless you dig out and clean out the entire area and chemically treat it. It's like having a dental teeth decay, the dentist needs to clean out the entire cavity "and" the surrounding areas, chemically treat it from spreading to other area. Not practical for these models.
Can the manufacturers mitigate this process? Boeing is having a difficult time with this in China. I hardly suspect that toy manufacturers will go through the process to ensure that they and the metal distributors are going to be ISO9000 compliant to ensure Standardization of Quality Control, Assurance and Management.
Sorry to ramble on and apologies in advance for the typo and grammar errors, it's long but, hope this information shed some light to the cause of "rot".