Brazing Handbook - Corrosion Resistance and Special brazing processes

Cleaning after brazing

Normally, no cleaning is needed after the brazing operation.

lnternal corrosion

The corrosion resistance of CuproBraze radiators is generally better than that of soldered radiators an d very competitive with that of aluminum.

When different metallic materials are used in the same cooling system, questions sometimes are raised about possible micro- galvanic corrosion risk, considering that the noble metal (copper) deposits on the unnoble metal (aluminum) surfaces. Inhibitor systems in the coolant are designed to prevent all kinds of corrosion in the cooling systems, including microgalvanic corrosion and, for this reason, the maintenance of the coolant is important. In general, however, copper alloys are less sensitive to a bad coolant than aluminum.

Corrosion test results in cool ants for CuproBraze materials (including tube brass SM 2385 and header brass SM 2464) are similar to test results for copper-based materials used in soft-soldered radiators. In a study on a mixed-metal cooling system, there was no indication of micro-galvanic corrosion on aluminum caused by copper. Therefore, the coolants th at fulfill the standard requirements with copper materials are considered compatible with materials used in CuproBraze radiators, and vice versa. CuproBraze heat exchangers are compatible with mixed-metal cooling systems.

External corrosion

The risks of external corrosion caused by galvanic attacks is minimized by the fact that the materials in CuproBraze radiators, including the copper-fin alloy, brass- tube alloy and the brazing alloy, have about equal mutual nobility. The brass-tube alloy contains 85 percent copper, which means that the alloy is less sensitive to stress corrosion cracking and dezincification than conventional brass alloys.

The brazing alloy OKC600 (CuNiSnP -type) also provides an extra protective and mechanically strengthening coating for tubes. Corrosion test results have been published. The references were soldered copper-brass radiators and brazed aluminum radiators. According to the results from four different kinds of accelerated corrosion tests, CuproBraze radiators were generally more corrosion resistant than soldered copper-brass radiators and very competitive with aluminum radiators. Generally, aluminum radiators were more prone to localized corrosion forms, whereas the corrosion form on CuproBraze was usually even and thus predictable.

Coatings

CuproBraze radiators are mechanically strong and facilitate thickness reductions High-performance coatings further improve corrosion resistance and make thickness reductions possible without risks from corrosion.

There are different ways to increase external corrosion resistance when reducing thickness. The easiest way is to leave the commonly used cosmetic coating off totally. An uncoated radiator has a lifetime that is about 30 percent longer than the lifetime of a cosmetically spray-coated radiator.

Electrophoretic coating is the best technical solution to increase corrosion resistance. It increase s the lifetime by 2.5 to 3 times compared to an uncoated radiator . A new option is powder coating with a multi-nozzle spray gun, which gives good results with respect to corrosion resistance and thermal performance and has a lower cost compared to an electrophoretic coating.

High-performance coatings cover the entire external radiator surface (not just 10 percent of it, as is the case for most conventional cosmetic-spray coatings). They clearly prolong the lifetime of the radiators. These technologies are commercially available.

One shot brazing

One shot brazing is when all or most of the joints in the heat exchangers are brazed simultaneously in one brazing cycle and there are no metallurgical differences between one shot and other brazed joints. If the brazing parameters for all joints are fulfilled, the joints will be satisfactorily brazed. There are some points to be noted. If the one-shot brazed heat exchanger includes inner fins or other kind of joints inside the component, it must be ensured that the atmosphere around all joints fulfill the recommendations for the atmosphere stated in chapter 6.1. In most cases, purging inside the heat exchanger with nitrogen is necessary. To ensure good brazing results in all joints, the joint geometry as well as fixturing and paste application should be designed for furnace brazing. During the brazing process some movements between parts in the joints can take place due to stress relieving in stamped parts and also due to differences in thermal expansion between the parts . When designing for one shot brazing th is should be taken into account. If possible, self-fixturing of the parts should be used.

The normal design for soldered tank-header joints, in most cases, has gaps that are too large for satisfactory brazed joints. The design for brazed joints should be in the recommendation earlier in this handbook, an optimal joint gap of 50 µm to a maximum 100 µm.

Figure 28 shows a norm a l geometry for soldering of header-tank. In figure 29 some improved header-tank designs are shown.

Figure 28. Typical joint for soldered header-tank. Gaps are too large

Figure 29. Alternative joint geometries for header-tank.

Many of the joints in “one shot” brazing are placed upside down or horizontally, it is therefore important to use a brazing paste suitable for this kind of joints.

The brazing paste should (if possible) be applied just outside the joints. During brazing the molten filler metal will b e drawn into th e joint by capillary force.

Brazing of parts with internal turbulators (CAC)

If fin tip application is used, fum e from the binder is form e d during the first part of the brazing cycle. Experience has shown that it is difficult to evaporate all fum e s from the inside of the tube s be fore the brazing takes place, consequently having a bad influence on the brazing result. Therefore to avoid the risk of scraping off the paste on the tips during assembling, brazing foil is recommended to be used as filler metal for internal turbulators, especially for charge air coolers (CAC). The foil can be inserted together with the turbulator s from the end of the tube, see figure 30.

Figure 30. Brazing foil used for internal fin brazing.

When turbulators similar to norm a l fins for heat exchangers are used, they should be designed to take up some elastic movements during the brazing cycle. If very stiff turbulators are used they can form a split in the sample after brazing.

Splitter-fin together w ith CuproBraze

Splitter-fin is a method to solder thin copper fins to a centre strip, which together forms a strong fin-module, which is easy to use and has improved heat performance (figure 31). The splitter-fin-module is so far not possible to produce with CuproBraze brazing-paste but soldered splitter-fins are sometimes used in CuproBraze heat exchangers. In this case the fin material is SM 0502 (the normal CuproBraze fin ma terial) and the soldered joints (lead -free) are used as “pre-joining). W hen t he soldered joints between the fins and the centre strip are heated up to brazing temperature, the joints will be transformed to a copper-tin phase with similar composition appearing at the brazed fins in the CuproBraze process.

Figure 31. Splitter-fin modules to the left and the module inserted in a core to the right.

Brazing of steel parts

Brazing any steel parts is not recommended, due to formations of brittle Fe-P-film between the filler and the base metal.