|  IP: Logged

If the oxidation is only on the area of the platter that was directly under the old film, it may be that moisture in the film was trapped, and affected the platter. I doubt any film-related effect would happen in only "the last couple weeks", unless the old acetate film had severe "vinegar syndrome" releasing free acetic acid. Do you smell any pungent/acid vapors from the old film roll?

What is the relative humidity in your projection room? --- excess moisture will cause rust and corrosion.

The relative humidity is around 52% and the oxidation(?) did not occur directly under the film, specifically. The film seems to be fine with no traces of vinegar syndrome or moisture.

 |  IP: Logged

Let us talk about aluminum and corrosion.

First, aluminum DOES NOT rust! Rust refers to ferrous (iron) alloys.
Aluminum corrodes.

The types of corrosion that effect aluminum and its alloys are:

a) Direct Surface Attack is the most common and results from the direct reaction of the metal surface with oxygen in the air and may be accelerated by salt spray or salt-bearing air, industrial gasses etc. Notice what a fingerprint does to a platter surface?

b) Dissimilar Metals Corrosion occurs when two dissimilar metals are in contact and are connected by an electrolyte.

c) Pitting is usually a localized breakdown of protection and can result from mechanical working, faulty heat treatment, an inclusion or localized contamination that breaks down surface protection.

d) Intergranular Corrosion is caused by improper heat treatment and grain boundries in the metal. Aluminum alloys 2024 and 7075 are highly vulnerable. I believe that platter decks are 2024. Exfoliation is an advanced form of this type of corrosion. The metal can blister and deliminate. If you look at a cross section of aluminum with intergranular corrosion the metal will be delamanated and resemble pages of a book.

e and f) Stress and Fatigue--self explanitory--

g) Fretting) is a rubbing contact that can destroy a protective coating and may remove virgin metal from the surface which could prevent the formation of a protective oxide. This is an abrasive action. also called False Brinnelling in relation to the hardness test. Maybe the film rubbing on the platter deck could have a similar result?

So how do we protect aluminum?

a) Anodizing. A good example of this will be the aluminum parts of the Christie P35-GPS that have a hard black coating.

b) Cladding. Pure aluminum is very corrosion resistant but has no structural strength. This is why we have alloys. Anyhow, cladding is when a paper thin layer of pure aluminum is hot rolled onto the surface of the alloy. Platter decks are not clad.

c) Painting and related surface treatments.

Do not clean platter decks with alcohol!!!
I have never experienced problems with 409. Manufacturers recommend cleaning platter decks with light soap and water solution followed by a rince with clean water and a thourough drying!

 |  IP: Logged

 |  IP: Logged