Corrosion Protection

Jul 28
2011
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Process related causes

The service life of a heat exchanger may be shortened due to corrosion caused by process related events. Some examples are listed below:

Excessively high brazing temperature or too long time at temperature will lead to excessive Si diffusion in the core. Si diffuses along grain boundaries and this can increase the susceptibility to intergranular corrosion. By maintaining proper time-temperature cycles and thereby minimizing Si diffusion, intergranular attack can also be minimized.

Copper in contact with aluminum will cause a corrosion related failure very quickly. Copper is noble (cathodic) to aluminum and when these two metals are in contact in the presence of an electrolyte, the aluminum will be consumed rapidly. This may occur in a heat exchanger manufacturing facility where both Al and Cu heat exchangers are produced and there is cross-contamination of process routes. It only takes one small Cu chip to land on the surface of Al during some part of the manufacturing process to cause a short-term failure in the Al heat exchanger. If both Al and Cu heat exchangers are to manufactured under the same roof, it is recommended (and practiced) to physically separate the two production routes with a wall and take extensive steps to avoid cross-contamination.

Carbonaceous residues can be generated on the heat exchanger surfaces during the heat cycle from residual lubricants, excessive use of surfactants, binders in flux or braze pastes etc. Carbon plays very much the same role as Cu in that it is noble to Al. In a corrosive environment, carbon residues act as a cathode and Al as an anode, leading to the galvanic corrosion of Al. The best preventative measure is to ensure that the heat exchangers are thoroughly and properly cleaned and degreased prior to brazing. This includes monitoring the flux slurry bath for any signs of organic contamination (for instance oil slicks).

Coatings

Painting a heat exchanger offers some level of corrosion protection, but is primarily used for cosmetic purposes. Painting will enhance corrosion protection if it covers the entire heat exchanger uniformly and is free from defects. In fact, paint defects or stone chips will accelerate corrosion locally. Many Al producers believe it is better to leave the heat exchanger unpainted to prolong its service life.

Conversion coatings such as chromate or phosphate conversion coatings work differently than painted surfaces. Conversion coatings enhance the natural oxide film on Al, essentially making it thicker and more resistant to hydrolysis. These types of coatings are most often used with automotive evaporators.

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Aluminium Core Alloys

May 21
2010
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Aluminum alloys are classified according to their alloying elements. The Aluminum Association designations are listed in the table below:

Designation System for wrought aluminum alloys
Alloys series   Description or major alloying element
1xxx                  99.00% minimum Aluminum
2xxx                 Copper
3xxx                 Manganese
4xxx                 Silicon
5xxx                 Magnesium
6xxx                 Magnesium and Silicon
7xxx                 Zinc
8xxx                 Other Element
9xxx                 Unused Series

The chemical composition of each AA alloy is registered by the Aluminum Association and a few examples are listed:

Example of aluminum alloy composition limits in weight percent*

Alloy-NoSiFeCuMnMgZnCrOther eachOther total
11000.95(Si + Fe)0.05 -0.200.05–-0.10–-0.050.15
14350.150.30 -0.500.020.050.050.10–-0.030.03
30030.600.700.05 -0.201.00 -1.50–-0.10–-0.050.15
30050.600.700.301.00 -1.500.20 -0.600.250.100.050.15
60630.20 -0.600.350.100.100.45 -0.900.100.100.050.15

*Maximum, unless shown as a range

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Brazing Aluminium and Copper

Mar 19
2010
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Brazing aluminium to copper is common in the refrigeration industry where copper tubes are brazed to aluminium roll-bond panels or tubes. To join aluminium and copper using brazing technology and standard NOCOLOK® Flux, flame brazing would be applicable (as well as using a low-melting flux with a low-melting filler metal). It is very similar to brazing aluminium to aluminium, but some precautions are necessary.

However, when copper is brazed to aluminium and the heating process takes too long, the copper will diffuse into the aluminium at the joints. A low melting Al-Cu alloy (Al-Cu33 eutectic temperature 548°C) is thus formatted, and this could lead to erosion by perforation.

Therefore, during the brazing process, the flame should never be directly applied to the joint, because the heat should be transferred by conduction through the parts to be brazed. As soon as the filler metal begins to melt, the flame must be quickly removed.

A second issue with brazing copper to aluminium is that the aluminium has a much lower melting point than copper (Al: app. 650°C; and Cu: above 1000°C). Therefore, the flame is usually directed on the copper. Nevertheless, once the heat transferred from the copper to the aluminium reaches the melting range of aluminium, it will start to burn down very fast, while the copper is still taking the heat. The formation of the above mentioned low melting Al-Cu alloy accelerates the destruction of the aluminium components.

Consequently, flame Brazing of aluminium to copper is a delicate process and requires some experience. But it is used by many companies for large scale production. But it is next to impossible in furnace brazing. There are no conventional furnace designs which will cool quickly enough to halt the continual formation of the aluminium-copper eutectic. For this reason, brazing copper to aluminium in a furnace is not practiced.

There are three different ways to provide (or generate) filler metal in flame brazing of aluminium to copper.

  • Use of Al-Si filler alloy (Al-Si 12 – AA4047). Standard procedure like in flame brazing of aluminium to aluminium – just a little bit faster to avoid burn-through.
  • Rely on the formation of Al-Cu alloy during the brazing cycle. If this method is used, a support provided by a thin Stainless Steel tube along the interior joint area can provide additional structural integrity.
  • A pre-heated copper tube is inserted very fast into an aluminium tube. The mechanical energy released will generate additional heat. Abrasion of surface oxide by the inserted tube promotes the formation of Al-Cu filler alloy. This process works with and without flux (however, results are better with flux).
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