News from our Sponsor Solvay Fluor:

NOCOLOK^® goes Smartphone!

NOCOLOK is a name synonymous with innovative products and solutions. No surprise then that Solvay Fluor is the first to provide a smartphone App for aluminium brazing. Comprehensive knowledge in a pocket-size format for all users in the aluminium industry is coming shortly. An absolute must-have for all smartphone users. The NOCOLOK App will be available for iOS and Android, providing a wealth of useful information all about brazing with NOCOLOK.

The App features a full listing of NOCOLOK products sorted according to application, comprehensive key data and the new GHS classifications. All NOCOLOK packaging units are clearly indicated with sizes and weights. The App comes complete with handy items, like a calculator for NOCOLOK quantities in various slurry concentrations and a tool to calculate heat exchanger surface areas with details of the required quantity of NOCOLOK in kg. And for those seeking more specifics, the NOCOLOK Encyclopedia presents information about aluminium brazing technology.

The NOCOLOK App is currently undergoing beta testing and will be presented soon.

We will inform you as soon as the App is available as a free download.

In its simplest form, a slurry is held in a reservoir tank and continuously agitated to prevent settling. The slurry is pumped, usually with air-diaphragm pumps to the flux slurry cabinet where the heat exchangers moving on a conveyor are sprayed with the slurry. After spraying, the excess flux slurry is blown off in a separate chamber with high volume air. The over spray and blown off slurry is recycled back to the reservoir tanks, again using air-diaphragm pumps.

Depending on the sophistication desired, a second flux spray chamber may be installed after the first chamber to deliver a higher concentration slurry to problem areas such as tube to header joints in condensers and radiators. This second spray chamber would have a separate flux delivery system and a separate reservoir tank to contain the higher concentration flux slurry.

The components of the flux delivery system including reservoir and agitators should all be constructed of stainless steel or chemically resistant plastics (nozzles for instance). There should be no mild steel or copper containing components – includes brass or bronze – in contact with the flux slurry. The schematic below shows the components of a generic fluxing station:

Note that splashing will occur inside the fluxing cabinet and cause an accumulation of dried flux on the walls. Therefore the cabinet is washed with water periodically to remove this accumulated flux. The frequency of this maintenance operation is up to the manufacturer, but could be anywhere from once per shift to once per month.

The general appearance of NOCOLOK^® brazed parts can range from relatively bright to light grey depending on the flux loading and furnace dew-point. When either is increased excessively over recommended levels, the appearance moves towards the grey colour. The flux residue usually can not be seen by the naked eye, however, it is visible under a microscope at 50x magnification. Higher magnification SEM views of the flux residue are shown in the pictures below.

The pictures on the left are typical of a tunnel furnace brazed surface, needle-like in structure possibly including the odd flat platelet. The pictures identified M-70323 are typical of a furnace atmosphere containing higher than recommended levels of O₂ specifically during the cooling cycle in a batch furnace. The morphology is almost 100% flat platelets. This surface has been reported to have better corrosion resistance in service.

Other flux residue properties are as follows:

a) Residue Thickness

Typically 1–2 microns. This can vary depending on flux coating weight prior to brazing.

b) Hardness

The residue hardness is about 4 on the Mohs scale.

c) Adhesion

No measurable loss has been found in circulation tests with freon or glycol type coolants using recommended flux loading. However there are reports that some detachment may occur where higher than recommended flux loading is used, particularly where molten flux pooled in downside areas.

d) Wettability

The post-braze flux residue has a hydrophilic (wetting) surface, however that wettability decreases with time.

e) Corrosion Resistance

The presence of flux residue on the part surface mildly increases corrosion resistance under normal conditions.

f) Solubility

Solubility of flux residue is influenced by the method of measurement. A typical value are between 1.2 and 3.0 g/l with Al, F and K ion concentrations approaching the stoichiometry of the compound KAlF₄ .

g) Post-Braze Odour

There is a slight odour from minute amounts of H₂S immediately after brazing. It disappears within a short time. If objectionable, the odour may be eliminated by rinsing the part with water.

h) Post-Treatment

The flux residue provides a good base for coatings. However, thicker residues resulting from higher than recommended flux loading can result in the poor or non-adhesion of wet or dry powder paint coatings.

NOCOLOK^® Flux residue is not easily removed from the surface of brazed parts. Mechanical abrasion, such as wire brushing or grit blasting, can be used to clean off heavier flux residues from “robust” joints. No practical chemical cleaning solution has been found. Boric acid and nitric acid solutions at higher temperature will partually remove the residues, however the times required (~ 1 hour) and the dangerous fuming with nitric acid preclude their use. Basically, the best procedure is to flux the product properly so that there is no visible after-braze residues and therefore no flux removal required.

Very often, heat exchanger manufacturers increase the flux loading on components to be brazed to compensate for furnace atmosphere or other process related deficiencies. The flux is an excellent “band-aid” and can be used as such, but only while the true problems are located and rectified. Long term use of higher than recommended flux loads can lead to other problems.

Over fluxing causes more KAlF₄ evaporation and condensation. This will load up the dry scrubber more quickly. White powder will accumulate more quickly on the curtains at the exit end of the furnace. If this is noticed, there is a very good chance that the dry scrubber is loading up more quickly.

There will be a more rapid build-up of the flux inside the furnace. This is a common issue with over fluxing whereby flux builds up on the muffle floor at the entrance to the cooling zone where it will solidify. This flux build up has been known to deflect the mesh belt.

There is more rapid build up of the flux on the fixtures which can significantly reduce maintenance intervals.

Over-fluxing can lead to visible flux residue on the brazed heat exchanger which may increase the incidence of flux residue fall-off. Excess flux residue dulls the appearance of a brazed heat exchanger and can also accumulate in the gasket areas causing problems with seals. Too much flux residue will also inhibit surface treatments such as painting or conversion treatments.

The theoretical amount of flux required to dissolve a 100 Å oxide film is about 0.02 g/m²
(1 Å = 10^-10 m = 0,1 nm). For a 400 Å film, still only 0.08 g/m² flux is required. These do not take into account losses to moisture, oxygen or poisoning of the flux by Mg alloy additions.

In practice however, the recommended loading for fluxing is 5 g/m², uniformly distributed on all active brazing surfaces. This is more than 250 times the theoretical amount required for oxide dissolution. To visualize what 5 g/m² flux loading might look like, think of a very dusty car. As the heat exchange manufacturer gains experience with his products, he may find that a little more is required for consistent brazing or that he can get away with a little less flux.

Too little flux will result in poor filler metal flow, poor joint formation, higher reject rates, and inconsistent brazing. In other words, the process becomes very sensitive.

Too much flux will not affect the brazing results. However there will be pooling of flux which can drip on the muffle floor, the surface of the brazed product will be gray and there will be visible signs of flux residue. Furthermore, flux will accumulate on fixtures more rapidly which then requires more frequent maintenance. More importantly yet, using too much flux will increase the process costs.

In some cases, heat exchanger manufacturers use higher than recommended flux loadings to mask furnace atmosphere deficiencies. This should be viewed as a short-term solution and the furnace problems should be addressed.

See also: How to evaluate flux load?

Based on currently available information, there is no simple cleaning method for flux residues by washing or dissolving – i.e. there is no suitable solvent or chemical solution – without attacking (corroding) the substrate material as well.

Mechanical Cleaning
Usually, removal of flux residue can only be done by mechanical means. From solid surfaces and from robust joints, as well as from Stainless Steel fixtures, the flux residues can be mechanically removed by sand or grit blasting. Wire brushing is a second alternative for flux residue removal. We recommend using Stainless Steel wire brushes for cleaning. Rotating SS-wire brushed are suitable too – provided the surface areas are accessible. Brushes made from copper and brass should only be used when the cleaned surfaces are not exposed to any subsequent welding or brazing cycle any more, because copper traces from the brush (even dust particles) in contact with aluminum can cause severe erosion problems.

Chemical Cleaning

Flux residue has a slightly higher solubility in strong alkalis and some acids. But in many cases the base materials (aluminum or Stainless Steel) will be attacked (corroded) by these chemicals too.

A solution of hot boric acid (10 to 15%, 75 – 80°C) can be used to remove some of the flux residue from brazed assemblies. Aluminum dissolution by boric acid is relatively moderate. The immersion time necessary to remove the bulk of residues varies from 10 to 30 minutes. But even then the flux residue removal will not be 100% successful.

Handling (preparation and usage) and discharging (waste disposal) of such chemical solutions can be problematic and expensive – due to their corrosive properties and the subsequently necessary waste water treatment. Considerations for health, safety and environment must be in accordance with the Safety Data Sheets.

Ultrasonic cleaning

Ultrasonic treatment may be effective in removing flux residues, provided that the parts to be cleaned fit into the ultra sonic dipping bath. A detergent (cleaning agent) can be added to the solution to improve the cleaning activity. The use of Antarox BL 225 for ultrasonic cleaning treatment is probably feasible. However, when there are any other additional chemicals mixed with the Antarox-containing cleaning solution (particularly when adding acids or alkalis) their compatibility with Antarox must be verified.

There are commercial solutions available for ultrasonic cleaning of Stainless Steel. More information on this subject is available from suppliers for industrial cleaning chemicals.

Summary

Flux residue from NOCOLOK Flux can only be removed by mechanical means, i.e. using wire brushes or grit/ sand blasting. This is a very difficult and laborious procedure – and a very dirty one (dust formation!). Local exhaust and ventilation is needed in the work area where the parts are cleaned. There is no suitable solvent to take off the flux residue without corroding the base materials.

Dust and dirt, condensates, lubricants and oils must be thoroughly removed. If the metal work pieces are poorly prepared, the flux will not spread evenly and the flow of filler alloy will be haphazard: it will either not spread properly or will discolour. The consequence would be an incomplete joint.

The first step is therefore: always clean the components of all oil and grease. The surfaces can be cleaned using either chemical, water-based or thermal cleaning techniques and substances.

Aqueous Cleaning

Aqueous or water based cleaning is a quite efficient and robust process, but still generates some waste water.

Aqueous cleaning starts off with a concentrated metal cleaning agent, which is subsequently diluted with water to 1% to 5% (v/v). The composition of a supplier’s cleaning solution is proprietary, but usually contains a mixture of surfactants, detergents and active ingredients such as sodium carbonate that serves to elevate the pH. Once diluted, the cleaning solution will typically have an elevated pH in the range of pH 9 to 12. There are acid based solutions, but appear to be less common.

The best water-based cleaners contain water, tensides, cleaning agent and active ingredients such as carbonates.

The cleaning solution works best at higher temperatures and is usually recommended to operate at 50°C to 80°C. Cleaning action is quicker at higher solution temperatures.

Thermal Degreasing

Thermal degreasing works by elevating the temperature of the work piece so that lubricants present on the surfaces will be evaporated. This procedure only works with special types of lubricants known as evaporative or vanishing oils. Vanishing oils are light duty lubricants used mostly for the fabrication of heat exchanger fins, although they are now finding uses in the stamping and forming of other heat exchanger components. Lubricants not designed for thermal degreasing must not be used. These could leave behind thermal decomposition products and carbonaceous residues which at higher level prevent brazing and have the potential to degrade product appearance and accelerate corrosion.

When brazing aluminum to stainless steel using:
a) NOCOLOK^® Flux and Al-Si filler alloys are suitable
or
b) alternatively CsAlF-Complex flux (melting range between 420 and 480°C) and Zn-Al filler alloys.

Regarding a): Brazing of aluminum to stainless steel works both with NOCOLOK^® Flux + Al-Si filler alloy and with NOCOLOK^® Sil Flux. After the flux melts and the oxides are removed, there is a reaction between Al and Fe, forming a thin intermetallic layer of FeAl₃. This layer forms the metallurgical bond between the Fe and Al components. FeAl₃ is very brittle and thus the thickness of this layer should be minimized, otherwise the joint can easily fracture.

From a metallurgraphic point of view, there is a multi-layer system (microscopic structures). First, there is the stainless steel, then the layer of FeAl₃, then the Al/Si filler metal, and finally the aluminum base material. The thickness of the brittle FeAl₃ layer is a function of brazing time and temperature; – consequently the need for a short brazing cycle with fast heat-up and very short holding time at maximum temperature. Too high brazing temperatures must be strictly avoided. Only with a short brazing cycle, successful joining of aluminum to steel is possible.

Joining of Al to steel using NOCOLOK^® Flux is done on large scale commercially for the production of pots and pans (stainless steel pots with aluminum ‘compensation base plates’) – mostly in induction brazing. It is also used for the production of heating elements (steel heating plates with aluminum base plates and tubes for the electrical heating wires). Another application for aluminum to steel joining is brazing of large aluminum-plated steel tubes – up to 11 meters long – with aluminum fins for power plant cooling modules.

In the manufacturing of pots and pans where there is a large surface area between the Al base plate and the pot, a mixture of filler metal powder and flux is often used. This circumvents the use of filler metal shim stock which is said to be costly and difficult to implement. In Al tube to steel or stainless steel tube joining, conventional flame brazing techniques can be used. Filler metal wire, either pre-placed or fed into the joint must be used. In the production of power plant cooling modules (with aluminum-plated steel tubes), the filler alloy is provided by clad fin material.

1) What is the NOCOLOK^® Sil Flux quantity (per cm²) required for sandwich brazing
or pressure cookers (stainless steel to aluminium)?
The recommended load for NOCOLOK^® Sil Flux is approximately 15 to 25 g/m². Brazing aluminium to stainless steel requires rapid processing, i.e. very fast heating ramp and short time at brazing temperature. Usually, this can only be accomplished with induction brazing.

When brazing aluminium to stainless steel using NOCOLOK^® Sil Flux, the Sil Flux first forms the filler metal from the aluminium component. The filler metal then reacts with the stainless steel to form a thin layer of FeAl₃.
From a metallurgraphic point of view, there is a multi-layer system (microscopic structures). First, there is the stainless steel, then the layer of FeAl₃, then the Al/Si filler metal, and finally the aluminium substrate. The FeAl₃ layer is very brittle, and so it is important that this layer is kept as thin as possible. The thickness of this layer is a function of time and temperature,- consequently the need for a short brazing cycle.

2) To prepare a NOCOLOK^® Sil Flux slurry or paste:
What is the exact mixing ratio (flux to solvent) required?
The mixing ratio for NOCOLOK^® Sil Flux slurries or pasts depends on the application method on site. In some cases, the main focus is a specific viscosity for an automated fluxing system. In other cases, only small flux quantities are prepared for immediate consumption.
NOCOLOK^® Sil Flux can be prepared with alcohol (ethanol or isopropyl alcohol) or alcohol/ water mixtures (70% alcohol content) in any ratio from 20 to 60 wt% (solids). As mentioned earlier, the actual slurry concentration will depend on the application procedure. The objective is to achieve 15 to 25 g/m² surface area.
If the NOCOLOK^® Sil Flux slurry is not completely consumed within one or two days, we recommend to use pure alcohol as carrier to avoid any chemical reaction between the solvent and the metal powder (silicon). Due to hydrolysis of the silicon powder, water should not be used to prepare NOCOLOK^® Sil Flux paste. Brushing, dipping or spraying can be utilised to apply the flux. Uniformity of the applied flux coating is very important.

3) How fast after applying NOCOLOK^® Sil Flux, the components should be 
processed for best results?
Before the part is heated up, the NOCOLOK^® Sil Flux slurry or past coating on the component surfaces should be thoroughly dried or allowed to evaporate. If alcohol is used as a carrier, the evaporation will only take a few seconds (with 15 to 25 g/m² flux load). NOCOLOK^® Sil Flux is non hygroscopic (i.e. the flux does not attract and absorb moisture) and non-corrosive under normal conditions (i.e. there is no reaction between the flux and the metal surfaces at room temperature). If a water mixture is used as the flux carrier, the components should be dried after flux application to avoid water-based corrosion effects.

4) What is the grain/particle size distribution of the silicon metal powder
in NOCOLOK^® Sil Flux?
The silicon particles in NOCOLOK^® Sil Flux show a particle size distribution curve with most of the grains within a range of 10 to 45μm. The Solvay specification for NOCOLOK^® Sil Flux coarse grade (which is used for brazing pots and pans) is as follows:
< 5μm: < 25%
10 to 20μm: 50%
> 35μm: < 10%

5) What are the key points regarding product fit-up for good joint formation
in NOCOLOK^® Sil Flux technology?
During the brazing process it is important that the components to be joint are in intermediate contact with each other. There must be firm pressure applied on all the surfaces of the plates throughout the brazing cycle to avoid large voids and gaps. Filler metal can only fill gaps up to a certain width (i.e. approximately 0.10 to 0.15 mm).