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Oxidation on Aluminium Pressure Die Castings

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Q. Dear Sir,

We are shipping pressure die castings in machined condition which are getting oxidized in transit . Oxidation is only on non-machined surface and never on machined surface

  

1 The grade chemical composition of the castings as LM-24 AlSi8Cu3 (Fe)(Zn) with the following chemical composition:

Copper 3 to 4
Mg 0.30 Max
Si 7.50 to 9.50
Fe 1.30 Max
Mn 0.50 Max
Ni 0.50 Max
Zn 3.00 Max
Lead 0.30 Max
Titanium 0.2 Max
Aluminium Balance

2 The mold release agent used is Water Soluble Die Lubricant having
Petroleum Oil 4.5%
High Pressure Additive 7.90%
Synthetic Oil < 1%
Synthetic Polymer Compound 3-4%
Emulsifier 1-2%
Biocide < 1-2%
Water : Balance
pH: 8-10%
Density 0.99 cm3
Evaporation Rate/Vapor Density/Vapor Pressure same as water.

3 Coolant used for machining is Servo Cut 'S' having the following details:

5% Oil mixed with water to form a white liquid used for cutting.

Castings are getting oxidized on un-machined surfaces only.

4 Packing is VCI Sheets/bags which prevents oxidation during sea shipment and then paper ply partitions inside a wooden case.

5.Castings are kept in open without packing at least one week after machining.This oxidation problem does not occur always but at frequent times, not in 100 % quantity but in part quantity.

6.Time that the castings will remain inside box is 30 days and on board of a ship is 21 days.
I request help to let us know why this oxidation and what should be done to prevent

Thanking you,