Brazing process Step 6: Cleaning the brazed joint
After you’ve brazed the assembly, you have to clean it. And cleaning is usually a two-step operation. First— removal of the flux residues. Second— pickling to remove any oxide scale formed during the brazing process.
Flux removal is a simple, but essential operation. (Flux residues are chemically corrosive and, if not removed, could weaken certain joints.) Since most brazing fluxes are water soluble, the easiest way to remove them is to quench the assembly in hot water (120°F/50°C or hotter). Best bet is to immerse them while they’re still hot, just making sure that the filler metal has solidified completely before quenching. The glass-like flux residues will usually crack and flake off. If they’re a little stubborn, brush them lightly with a wire brush while the assembly is still in the hot water.
Depending on your brazing process, you may need to perform post-braze joint cleaning to remove residual flux. This step can be crucial since most fluxes are corrosive, such as the pictured refrigeration line corrosion.
Reasons to Remove Flux
Let's examine five reasons why post-braze flux removal is important:
- You cannot inspect a joint that is covered with flux.
- Flux can act as a bonding agent and may be holding the joint together, without successful brazing. This joint would fail during service.
- In pressure service, flux may mask pinholes in a braze joint, even though it withstands a pressure test. The joint would leak soon after being placed into service.
- Flux is hygroscopic, so residual flux attracts available water from the environment. This leads to corrosion.
- Paint or other coatings do not stick to areas covered with residual flux.
Methods for Flux Removal
After brazing, flux forms a hard, glass-like surface and can be difficult to remove. What is the best cleaning method? You can remove excess flux by various means; the most cost-effective approaches involve water.
Industry flux standards focus on water-based fluxes. AMS 3410 and AMS 3411 stipulate that all fluxes conforming to these specifications should be soluble in water at 175°F/79°C or less after brazing. Therefore, brazing fluxes are typically designed to dissolve in water.
The most common methods for post-braze flux removal are:
Use hot water with agitation in a soak tank to remove excess flux immediately following the braze operation, and then dry the assembly. When soaking is not possible, use a wire brush along with a spray bottle or wet towel. When using a soak bath of any kind, change the solution periodically to avoid saturating the cleaning solution.
This process induces a thermal shock that cracks off residual flux. When quenching a brazed part in hot water, take care to avoid compromising the braze joint. Quench only after the braze filler metal has solidified to avoid cracks or rough braze joints. Note that quenching can affect base material mechanical properties. Do not quench materials with large differences in coefficients of thermal expansion to avoid cracks in the base materials and tears within the braze alloy.
You can use more elaborate methods of removing flux as well—an ultrasonic cleaning tank to speed the action of the hot water, or live steam. Additional cleaning methods include:
- Steam lance cleaning - This process employs superheated steam under pressure to dissolve and blast away flux residue.
- Chemical cleaning - You can use an acidic or basic solution, generally with short soak times to avoid deteriorating the base materials. For chemical soaks, monitor the pH level to determine when to change the solution.
- Mechanical cleaning - Clean residue from brazed joints with a wire brush or by sandblasting. Be advised that soft metals-including aluminum-require extra care, as they are vulnerable to the embedding of particles.
Always ensure that your cleaning method is compatible with base metal properties. Some metal groups achieve a desired effect from a special treatment after cleaning. Stainless-steel and aluminum parts, for example, may benefit from chemical immersion to improve surface corrosion resistance.
The only time you run into trouble removing flux is when you haven’t used enough of it to begin with, or you’ve overheated the parts during the brazing process. Then the flux becomes totally saturated with oxides, usually turning green or black. In this case, the flux has to be removed by a mild acid solution. A 25% hydrochloric acid bath (heated to 140-160°F/60-70°C) will usually dissolve the most stubborn flux residues. Simply agitate the brazed assembly in this solution for 30 seconds to 2 minutes. No need to brush. A word of caution, however—acid solutions are potent, so when quenching hot brazed assemblies in an acid bath, be sure to wear a face shield and gloves.
After you’ve gotten rid of the flux, use a pickling solution to remove any oxides that remain on areas that were unprotected by flux during the brazing process. The best pickle to use is generally the one recommended by the manufacturer of the brazing materials you’re using. Highly oxidizing pickling solutions, such as bright dips containing nitric acid, should be avoided if possible, as they attack the silver filler metal. If you do find it necessary to use them, keep the pickling time very short.
Recommended pickling solutions for post-braze removal of oxides
Oxide removal from copper, brass, bronze, nickel silver and other copper alloys containing high percentages of copper.
|10 to 25% hot sulphuric acid with 5-10% potassium dichromate added.
||Pickling can be done at same time flux is removed. Will work on carbon steels, but if pickle is contaminated with copper, the copper will plate out on the steel and will have to be removed mechanically, This sulphuric pickle will remove copper or cuprous oxide stains from copper alloys. It is an oxidizing pickle, and will discolor the silver filler metal, leaving it a dull gray.
|Oxide removal from irons and steels.
||A 50% hydrochloric acid solution, used cold or warm. More diluted acid can be used (10-25%) at higher temperatures (140-160°F/60-70°C.)
||A mixture of 1 part hydrochloric acid to 2 parts water can be used for Monel and other high nickel alloys. Pickling solution should be heated to about 180°F/80°C. Mechanical finishing is necessary for bright finishes. This HCl pickle is not like bright dips on nonferrous metals.
|Oxide removal from stainless steels and alloys containing chromium.
||20% sulphuric acid, 20% hydrochloric acid, 60% water, used at 170-180°F (75-80°C.)
||This pickle is followed directly by a 10% nitric dip, and then a clean water rinse.
||20% hydrochloric acid, 10% nitric acid, 70% water, used at about 150°F (65°C.)
||This pickle is more aggressive than the sulphuric-hydrochloric mixture listed above, and will etch both the steel and the filler metal.
Note: The pickles recommended above will work with any of the standard silver filler metals, and no specific instructions are required for the individual filler metals. The phos-copper and silver-bearing phos-copper filler metals are different, and then only when used on copper without flux. In this case, a hard copper phosphate slag forms in small globules on the metal surface. Prolonged pickling in sulphuric acid will remove this slag, but a short pickle in 50% hydrochloric acid for a few minutes is more effective. When the brazed joint is to be plated or tinned, the removal of the slag is absolutely essential. A final mechanical cleaning, therefore, is advisable for work which is to be plated.
Post-Cleaning Inspection of Brazed Joints
Depending on your brazing process, you may need to perform post-braze joint cleaning to remove residual flux. This step is crucial for several reasons; including the corrosive nature of most fluxes and the possibility that excess flux could contribute to joint failure. The most common cleaning methods involve water-specifically soaking/wetting and quenching.
Discontinuities During Joint Inspection
Examining finished joints may be the final step in the brazing process, but inspection procedures should be incorporated into the design stage. Your methodology will depend on the application, service and end-user requirements plus regulatory codes and standards.
Define your acceptance criteria for any discontinuity with considerations for shape, orientation, location (surface or subsurface) and relationship to other discontinuities. Be sure to state acceptance limits in terms of minimum requirements.
Common discontinuities of brazed joints, identified through nondestructive examination, include:
- Voids or porosity - an incomplete flow of brazing filler metal which can decrease joint strength and allow leakage-often caused by improper cleaning, incorrect joint clearance, insufficient filler metal, entrapped gas or thermal expansion.
- Flux entrapment - resulting from insufficient vents in the joint design-preventing the flow of filler metal and reducing joint strength as well as service life
- Discontinuous fillets - areas on the joint surface where the fillet is interrupted-usually discovered by visual inspection
- Base metal erosion (or alloying) - when the filler metal alloys with the base metal during brazing-movement of the alloy away from the fillet may cause erosion and reduce joint strength
- Unsatisfactory surface condition or appearance - excessive filler metal or rough surfaces-may act as corrosion sites and stress concentrators, also interfering with further testing
- Cracks - reducing strength and service life of the joint-may also be caused by liquid metal embrittlement.
Brazed Joint Examination Methods: Nondestructive testing methods
Nondestructive testing methods of checking quality and specification conformance include:
Visual examination - with or without magnification-for evaluating voids, porosity, surface cracks, fillet size and shape, discontinuous fillets plus base metal erosion (not internal issues such as porosity and lack of fill)
Leak testing - for determining gas- or liquid-tightness of a brazement. Pressure (or bubble leak) testing involves the application of air at greater-than-service pressures. Vacuum testing is useful for refrigeration equipment and detection of minute leaks, employing a mass spectrometer and a helium atmosphere.
Radiographic examination - useful in detecting internal flaws, large cracks and braze voids, if thickness and X-ray absorption ratios permit delineation of the brazing filler metal-cannot verify a proper metallurgical bond (pictured right)
Proof testing - subjecting a brazed joint to a one-time load greater than the service level-applied by hydrostatic methods, tensile loading or spin testing
Ultrasonic examination - a comparative method for evaluating joint quality, in immersion mode or contract mode-involves reflection of sound waves by surfaces, using a transducer to emit a pulse and receive echoes (pictured to the right)
Liquid penetrant examination - dye and fluorescent penetrants may detect cracks open to the surface of joints-not suitable for inspection of fillets, where some porosity is always present
Acoustic emission testing - evaluating the extent of discontinuity-using the premise that acoustic signals undergo a frequency or amplitude change when traveling across discontinuities
Thermal transfer examination - detects changes in thermal transfer rates due to discontinuities or unbrazed areas-images show brazed areas as light spots and void areas as dark spots
Brazed Joint Examination Methods: Destructive testing methods
There are also several destructive and mechanical testing methods, often used in random or lot testing:
Peel testing - useful for evaluating lap joints and production quality control for general quality of the bond plus presence of voids and flux inclusions-where one member is held rigid while the other is peeled away from the joint
Metallographic examination - testing the general quality of joints-detecting porosity, poor filler metal flow, base metal erosion and improper fit
Tension and shear testing - determines strength of a joint in tension or in shear-used during qualification or development rather than production
Fatigue testing - testing the base metal plus the brazed joint-a time-consuming and costly method
Impact testing - determines the basic properties of brazed joints-generally used in a lab setting
Torsion testing - used on brazed joints in production quality control-for example, studs or screws brazed to thick sections
Failed Brazing Inspection
The size, complexity and severity of the application determine the best inspection method, and several methods may be required. If you are unable to develop an accurate and dependable method of inspecting a critical brazed joint, consider revisiting your joint design to allow adequate inspection.
Examining finished joints may be the final step in the brazing process, but inspection procedures should be incorporated into the design stage. Both nondestructive and destructive methods may be employed, depending on the application, service and end-user requirements plus regulatory codes and standards.
Once the flux and oxides are removed from the brazed assembly, further finishing operations are seldom needed. The assembly is ready for use, or for the application of an electroplated finish. In the few instances where you need an ultra-clean finish, you can get it by polishing the assembly with a fine emery cloth. If the assemblies are going to be stored for use at a later time, give them a light rust-resistant protective coating by adding a water soluble oil to the final rinse water.
Watch this video for more on how to properly clean joints.