In airplane maintenance, what does the term corrosion mean?

This document proposes to require that the maintenance or inspection programs for all airplanes operated under part 121 of Title 14, Code of Federal Regulations, all U.S.-registered multiengine airplanes operated in common carriage by foreign air carriers or foreign persons under 14 CFR part 129, and all multiengine airplanes used in scheduled operations under 14 CFR part 135 include FAA-approved corrosion prevention and control programs. Such programs are needed because existing maintenance and inspection programs may not provide comprehensive, systematic measures to prevent and control corrosion. These proposals form a part of the FAA’s response to legislation emanating from the Aging Aircraft Safety Act of 1991. These actions are intended to control the detrimental effects of corrosion and the resulting airplane structural material loss.
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DEFINITION OF CORROSION. Corrosion is
the electrochemical deterioration of a metal because of its
chemical reaction with the surrounding environment. This
reaction occurs due to the tendency of metals to return to
their naturally occurring states, usually oxide or sulfide
ores. For example, iron in the presence of moisture and air
will return to its natural state, iron oxide or rust. Aluminum
and magnesium form corrosion products that are
white oxides or hydroxides. When a water solution containing
soluble salts is present, corrosion of many alloys
can occur easily at ambient temperatures. This type of
corrosion can be effectively treated by maintenance personnel
as discussed in this manual. Corrosion can also
occur in the absence of water but only at high temperatures,
such as those found in gas turbine engines. However,
the most common type of corrosion (and the one that
can be most effectively treated by maintenance personnel)
is electrochemical corrosion.

THEORY OF CORROSION. All structural metals
will corrode to some extent in a natural environment.
When a metal corrodes, the metal atoms lose electrons and
become positively charged metal ions in the electrolyte. In
solution, the positively charged metal ions can combine
with negatively charged ions to form corrosion products,
such as metallic chlorides, oxides, hydroxides, sulfides,
etc. Four conditions must exist before this type of corrosion
can occur.
a. A metal which has a tendency to corrode must be
present (the corroding metal is known as the anode);
b. A dissimilar conductive material (the cathode)
which has less tendency to corrode than the anode must be
present (such as a different metal, a protected part of the
same metal, or conductive plastics);
c. A conductive liquid (electrolyte) must connect the
anode and cathode (so that ions can carry electric current
between them); and
d. Electrical contact between the anode and cathode
(usually in the form of metal–to–metal contact) must exist
(so that electrons can move from the anode, where they
are released, to the cathode).

Galvanic corrosion. Galvanic corrosion occurs
when different metals are in contact with each other and
an electrolyte, such as salt water. It is usually recognizable
by the presence of a buildup of corrosion at the joint between
the metals. For example, aluminum skin panels and
stainless steel doublers, riveted together in an aircraft
wing, form a galvanic couple if moisture and contamination
are present. Figure 2–6 shows galvanic corrosion of
magnesium adjacent to steel fasteners. When metals which
are known to be in electrical contact are well separated
from each other in Table 2–1 galvanic corrosion is probably
occurring.
2–7.3. Pitting corrosion. The most common corrosion on
aluminum and magnesium alloys is called pitting (see Figure
2–7). It is first noticeable as a white or gray powdery
deposit, similar to dust, which blotches the surface. When
the deposit is cleaned away, tiny pits or holes can be seen
in the surface (see Figure 2–8). Pitting corrosion can also
occur in other types of alloys. The combination of small
active anodes to large passive cathodes cause severe pitting.
2–7.4. Intergranular corrosion. Intergranular corrosion is
an attack on the grain boundaries of the metal. A highly
magnified cross section of any commercial alloy (see Figures
2–9 and 2–10) shows the granular structure of the
metal. It consists of quantities of individual grains, each
having a clearly defined boundary, which chemically differs
from the metal within the grain. Frequently the grain
boundaries are anodic (tend to corrode more easily) to the
metal within the grain. When in contact with an electrolyte,
rapid corrosion occurs at the grain boundaries. Figure
2–11 shows intergranular corrosion of 7075–T6 aluminum
alloy adjacent to steel fasteners. In this example, the grain
boundaries are anodic to both the metal grain and the steel
fastener.
References :
USAF Corrosion manual

The manuals for each aircraft detail how much corrosion is acceptable before repairs must be done. Aluminium does corrode in air but one must also take into account what other chmicals can do to the metal.
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Yes, the forming of a white powder is one indication of aluminum corrosion. (Others would be pitting, flaking, or wearing away, but that’s not germane to your question.) Light corrosion damage is acceptable; the aircraft manufacturer will determine the allowable damage.

Once corrosion is identified, as long as damage limits are not exceeded, action will be taken to prevent more damage. This usually entails removal of corrosion products, chemical treatment (‘Alodine’ being one example) to ‘harden’ the surface, then priming and painting to prevent further intrusion by corrosion causing material.

Corrosion damage exceeding the allowable limits will be removed and replaced.
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15+ years aircraft mechanic.