Case Study

A radiator core retrieved from service was examined for a suspected premature corrosion related failure.
Upon closer metallographic examination, no evidence of corrosion was found at the failed area.

33% tube core erosion in the failure area

Tube: AA4343/ AA3003
 
 
 
 
 
 
 
 
It was concluded that the cause of the failure was in fact a mechanical failure occuring in the thinned wall area.

The following sequence of events proposes a rational explanation for the eroded tube area:

In service radiators are subject to internal pressure fluctuations and expansion and contraction due to heating and cooling. Mechanical failure was imminent and occured in the weakest part of the tube, the thinned down tube wall area adjacent to the tube to header joint.

Conclusions

Erosion of the base metal is undesirable since it reduces the wall thickness of the brazed component.
In addition Si penetration in the grain boundaries is known to increase the susceptibility to intergranular corrosion. Therefore proper filler metal management practices should be observed to prevent undesirable effects. One such factor easily controlled by the brazer is maximum peak brazing temperature.

Experimental

The effect of temperature on filler metal erosion was studied using an automotive radiator core.

610 °C for 2 minutes - no thinning of the tube core

625 °C for 2 minutes - significant erosion of the tube core

In this case, joining progresses initially as expected. The cladding layer on the tube melts and flows by capillary action to the fin to tube joint and a normal fillet forms. However, as the peak brazing temperature is allowed to rise beyond the recommended maximum (605 °C) the following occurs:

• The fluidity of the filler metal at the tube to header joint is increased and some of the liquid filler metal is released and flows to the nearest tube to fin joints.
• Excess filler metal at the tube to fin joints accelerates dissolution of the tube core adjacent to the fin, eroding the tubewall thickness.
• The excess filler metal pool is then drawn by capillary action in between the fins, particularly where the finspacing is narrow. The fins are drawn together by the strong capillary forces, displacing the fin from its original fin to tube position.
• As the fins move together, drawing the filler metal pool from its original position, the denuded area is significantly reduced in cross sectional thickness.

In some instances the extent of filler metal erosion is so severe that the entire thickness of the tube is consumed resulting in catastrophic failures.

More about this topic in our next issue.

The European Association for Brazing and soldering — EABS for short — together with experts from Solvay Fluor, holds technical training seminars in which the theory and practice of flame and furnace aluminium brazing are communicated in detail.

40 interested participants from all over the world gather for the two day seminar in Hannover, Germany: technical staff, design and production engineers as well as production engineering managers.

EABS Seminar

More information about the seminar.

HF can potentially be formed during the flux brazing process. HF is very toxic, irritating to the eyes, skin and respiratory tract and cause severe burns of the skin and eyes. The threshold limit value (TLV) for HF is a ceiling concentration of 3 ppm (2.3 mg/m³), a concentration that should not be exceeded during any part of the working shift.

Drying ovens can be electrically heated or gas fired. In gas fired drying ovens, it is possible that any flux particles entrained in the moist air and passed through the high temperature flames may generate HF. The concern here is not so much with employee exposure, but that HF may be released into the atmosphere.

Similarly, flux particles coming in contact with the hot flames in a flame brazing station may also generate HF. Suitable local exhaust systems must be in place to capture vapors and fumes that may contain HF.

It is known that one of the components of the flux, KAlF₄, has a measurable vapor pressure and the rate of evaporation increases rapidly once the flux is molten. With regard to CAB brazing (furnace brazing) where traces of moisture are always present even at below –40°C dew point, a number of compounds can be formed in the system K – Al – F – H – O. To our knowledge there has been no academic effort to create a thermodynamic model of this system. Thus, it is impossible to predict which compounds will and will not exist, and in what temperature or humidity regimes. This is why more than one mechanism has been proposed for the generation of HF, but no unique reaction mechanism has been identified:
3KAlF₄ + 3H₂O → Al₂O₃ + K₃AlF₆ + 6HF
2KAlF₄ + 3H₂O → 2KF + Al₂O₃ + 6HF

While the evidence above points to gas phase reactions between flux fumes and water vapor for the generation of HF, Thompson and Goad¹⁾ proposed that AlF₃ dissolved in the flux melt is subject to hydrolysis according to:

2AlF₃ + 3H₂O → Al₂O₃ + 6HF

What is clear is that in all cases, HF is shown as a reaction product. As for the quantity, Field and Steward²⁾ have indicated that the amount of HF formed is typically 20 ppm in the exhaust of a continuous tunnel furnace. Solvay’s own research work showed that even when flux on aluminum is heated in a bone-dry nitrogen atmosphere, a small quantity of HF is still generated³⁾. A source of hydrogen must be made available for HF to be formed even under bone-dry conditions and this might include reduction of aluminum hydroxide, degassing of furnace walls, leakage or other less obvious sources. The work showed that even under ideal conditions, it is virtually impossible to avoid some HF formation. The graph below shows the relationship between dew point and HF formation:

The amount of HF generated depends on several factors such as:

• Flux load going through the furnace – flux loading and component throughput
• Temperature profile – heating rate and time at temperature
• Furnace atmosphere conditions such as nitrogen flow and dew point

The HF is exhausted together with the nitrogen stream and absorbed by the dry scrubber.

NOCOLOK is a name synonymous with innovative aluminium brazing flux products and solutions. So it’s no surprise that Solvay Fluor is the first to provide a smart phone App for aluminium brazing. Comprehensive knowledge in pocketsize format for all users in the aluminium industry is now available free of charge in the App Store and the Android Market under the name NOCOLOK. A new video shows the advantages of the NOCOLOK App.

The new version 1.1 is now compatible with the iPad and uses the full resolution.

Get the NOCOLOK App for the iPhone and the iPad:
itunes.apple.com/de/app/nocolok/id474723690?mt=8&ls=1

Get the NOCOLOK App for Android Smartphones:
https://market.android.com/details?id=de.ahlersheinel.nocolok

First App of its kind worldwide:
NOCOLOK Flux brings aluminium brazing know-how to your mobile phone

The innovative producer of NOCOLOK Flux brazing flux, and world market leader Solvay Fluor, now bundles all the information on aluminium brazing in a smartphone App. The new NOCOLOK Flux-App puts all the information you need for your day-to-day business right where you need it – at your fingertips. This is another logical move in Solvay Fluor’s strategy of providing a full service on all aspects of aluminium brazing.

The App boasts a detailed product overview of all NOCOLOK fluxes and ancillary products, as well as their physical properties and GHS classifications. From brazing, soldering, powder and paint flux  coating to perfect corrosion protection: the App lists the packaging units, together with their weights, dimensions and a picture to simplify the selection of the required product.

But the real highlights of the application are the two calculators – which really help your day-to-day routines: the “Flux Slurry Calculator” calculates the amount of NOCOLOK Flux needed dependent on the number of litres required and the concentration of the slurry. The “Flux Load Calculator” calculates the surface of a heat exchanger and the amount of flux required for the brazing process. Additionally detailed answers on fundamental aspects and special features of aluminium brazing with NOCOLOK are provided at a touch by the “NOCOLOK Encyclopaedia”.

The free English-language iPhone-App (from iOS 4.0) is available now at the App Store for download. The Android version is available at Android Market.  The iPad version will be available shortly.

Just scan the QR code with your smartphone to get the NOCOLOK App:

OR Code for NOCOLOK App iOS

OR Code for NOCOLOK App Android

How to obtain?

A lot of information can be gained from heat exchanger brazing cycle temperature profile. It is probably one of the most important pieces of information that the brazing engineer can use to fully understand his process. A temperature profile will provide information such as heating rate, maximum peak brazing temperature, time at temperature, temperature uniformity across the heat exchanger and cooling rate. No other tool can provide so much information.

The simplest method for obtaining a temperature profile is to attach thermocouple wires to various parts of the heat exchanger and graphing the resulting profile on a chart recorder. The disadvantage of this method is that the thermocouple wire must be long enough to traverse the length of the furnace. One must also ensure that the wire does not become entangled in the mesh belt.

The second and more common (also more expensive) method of obtaining temperature profiles is with the use of a thermally insulated data pack. The data pack is a stand-alone unit capable of withstanding brazing temperatures. The thermocouples wired into the data pack are attached to various parts of the heat exchanger. The data pack then travels on the belt with the heat exchanger through the brazing furnace. At the end of the run, the data stored in the data pack is downloaded into a computer where graphs can be generated. The sophisticated software allows the user to determine quickly a number of parameters such as maximum temperature reached by each thermocouple.

Recent advances in thermal profiling allows getting information in real time. The thermally insulated data pack transmits data in real time from inside the brazing furnace to a computer situated outside the furnace using the latest radio telemetry technology. Changes to the furnace settings can now be seen instantly¹.

Heating Rate

A minimum average heating rate of 20°C/min up to the maximum brazing temperature is recommended. With very large heat exchangers such as charge air coolers, lower heating rates may be used, but with higher flux loadings. Once the flux starts to melt, it also begins to dry out. With slower heating rates, it is possible that the flux can be sufficiently dry as to loose its effectiveness when the filler metal starts to melt or before the maximum brazing temperature is reached.

Heating rates up to 45°C/min in the range of 400°C to 600°C are not uncommon. One could say that the faster the heating the better. However, temperature uniformity across the heat exchanger must be maintained especially when approaching the maximum brazing temperature and this becomes increasingly more difficult with fast heating rates.

Maximum Brazing Temperature

For most alloy packages, the recommended maximum peak brazing temperature is anywhere from 595°C to 605°C and in most cases around 600°C.

Temperature Uniformity

During heat up, there may be quite a variation in temperature across the heat exchanger. The variation will tighten as the maximum temperature is reached. At brazing temperature it is recommended that the variation should not exceed ± 5°C. This can be difficult to maintain when larger units are processed which have differing mass areas within the product.

Time at Temperature

The brazed product should not remain at the maximum brazing temperature for any longer than 3 to 5 minutes. The reason is that a phenomenon known as filler metal erosion (core alloy dissolution / Silicon penetration into the base material) begins to take place as soon as the filler metal becomes molten. And so the longer the filler metal remains molten, the more severe the erosion is.

The graph below shows an actual temperature profile for a heat exchanger brazed in a tunnel furnace. One characteristic feature of all temperature profiles is where the curve flattens out when approaching the maximum peak brazing temperature (area shown in blue circle). The plateau in the temperature profile is associated with the start of melting of the filler metal at 577°C, known as the latent heat of fusion. It is called latent heat because there is no temperature change when going from solid to liquid, only a phase change.

Temperature profile for a heat exchanger brazed in a tunnel furnace.

¹ D. Plester, Datapak Ltd., International Congress Aluminium Brazing, Düsseldorf (2002)

Furnace atmosphere

The recommended furnace atmosphere conditions necessary for good brazing are as follows:

• Dew point: ≤ -40°C
• Oxygen: < 100 ppm
• Inert gas: nitrogen

The most common source of nitrogen is that generated from liquid nitrogen storage tanks. A typical nitrogen gas specification from a liquid source indicates that the moisture content is <1.5 ppm (dew point = -73°C) and an oxygen level of <3 ppm. In brazing furnaces however, the normal atmospheric operating conditions almost always exceed incoming nitrogen contaminant levels. This is due to water and oxygen dragged into the furnace by the incoming products, by the stainless steel mesh belt and by the potential back-streaming of factory atmosphere through the entrance and exit of the furnace. The latter will occur when the exhaust and incoming nitrogen are not properly balanced.

Many furnaces are equipped with dew point and oxygen measurement devices. It is important that the measurements are taken in the critical brazing zone of the furnace because this is where these impurities will reach their lowest concentrations. Measuring dew point or oxygen levels anywhere else in the furnace may be of academic interest, but will not represent actual brazing conditions.

Dew Point Measurement

Measuring the moisture content in the critical brazing zone of the furnace has always been a key indicator of the quality of the brazing atmosphere. Moisture can substantially influence the quality and appearance of the brazed heat exchanger as well as the first time through braze quality (% rejects).

Chilled Mirror Technology

One of the more common principles of measuring dew point is using chilled mirror technology. The measurement of the water vapor content of a gas by the dew point technique involves chilling a surface, usually a metallic mirror, to the temperature at which water on the mirror surface is in equilibrium with the water vapor pressure in the gas sample above the surface. At this temperature, the mass of water on the surface is neither increasing (too cold a surface) nor decreasing (too warm a surface).

In the chilled-mirror technique, a mirror is constructed from a material with good thermal conductivity such as silver or copper, and properly plated with an inert metal such as iridium, rubidium, nickel, or gold to prevent tarnishing and oxidation. The mirror is chilled using a thermoelectric cooler until dew just begins to form. The temperature at which dew is formed on the mirror is displayed as the dew point.

The advantage of the chilled mirror dew point meter is that it is an absolute measurement with high precision. However, this measurement technique is sensitive to pollutants and corrosive contaminants which, in the brazing process, include KAlF4 condensation and trace amounts of HF gas. Consequently, the mirror requires frequent maintenance and replacement. “Dirty” mirrors can lead to false readings.

Coulometric Measurement Principle

The principle of operation for measuring is that an electrolyte is formed by absorption of water on a highly hygroscopic surface (e.g. P₂O₅) and the current level obtained to electrolyze the surface is proportional to the water content. The advantage of this principle of operation is that it is insensitive to aggressive media. The disadvantage is that the precision is not as high as chilled mirror technology. Some heat exchanger manufacturers have reported good success using this measurement principle in their CAB furnaces.

Relationship between dew point and moisture content

The relationship between dew point and moisture content is not linear. It is important to note that small changes in dew point will result in large changes in actual moisture content. This is evident from the graph shown below.

As manufactured, a non-corrosvie K-Al-F-type flux typically is a mixture of potassium tetra-fluoroaluminate (KAlF₄), and also contains potassium penta-fluoroaluminate (K₂AlF₅). K₂AlF₅ exists in different modifications: potassium penta-fluoroaluminate hydrate (K₂AlF₅ · H₂O), and hydrate-free (K₂AlF₅).
During the brazing process, the material undergoes essential physico-chemical alterations. While the chief component, KAlF₄, is simply heated up, the compound K₂AlF₅ · H₂O begins to lose its crystal water from 90°C (195°F) on. When the temperature is further increased within the ranges of 90° – 150°C (195°F – 302°F), and 290°C – 330°C (554°F – 626°F), two different crystallographic (structural) modifications of K₂AlF₅ are formatted.

When the furnace temperature is raised above 490°C (914°F), K₂AlF₅ begins to react chemically. According to the equation:

2 K₂AlF₅ → KAlF₄ + K₃AlF₆ (Equation 1)

the exact amount of potassium hexa-fluoroaluminate (K₃AlF₆) necessary for a eutectic flux composition (i.e. mixture of two or more substances which has the lowest melting point; see phase diagram) is obtained from the original K₂AlF₅ content. At brazing temperature, the resulting flux composition has a clearly defined melting range of 565°C to 572°C (1049°F – 1062°F). The flux melts to a colorless liquid.
Due to a vapor pressure of 0.06 mbar at 600°C, some of the KAlF₄ evaporates during the brazing cycle, particularly once melting temperature is reached. The total content of KAlF₄ contained in the exhaust is depending on time and temperature. Based on results from TGA analysis (with a heating rate of 20°C/min), the quantity of volatile compounds in Flux between 250°C and 550°C (482°F and 1022°F) is approximately 0.2 to 0.5%. These flux fumes contain fluorides and have the potential to react with the furnace atmosphere, especially moisture, to form hydrogen fluoride according to the equation:

3 KAlF₄ + 3 H₂O → K₃AlF₆ + Al₂O₃ + 6 HF (Equation 2)

This is one of the reasons, why the brazing process should take place in a controlled atmosphere (nitrogen) with low dew point and low oxygen level (another reason is to minimize re-oxidization effects on the aluminum surfaces).

Directly after brazing has been completed, flux residues consist mainly of KAlF₄ and K₃AlF₆. In the presence of moisture from the surrounding atmosphere, the K₃AlF₆ is converted back to K₂AlF₅ · H₂O over time (several days) in a reaction reverse to the one described in equation 1 followed by a re-hydration step.

The schematic below illustrates the transformations that occur as the flux is heated to brazing temperature. Note that these phases are unstable outside the furnace atmosphere.

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 di