Making Corrosion-Resistant Brazed Joints in Stainless Steel ADVANTAGES of PALLADIUM-BEARING BRAZING ALLOYS

Making Corrosion-Resistant Brazed Joints in Stainless Steel ADVANTAGES OF PALLADIUM-BEARING BRAZING ALLOYS

By M. H. Sloboda, Dipl. Ing. Research Laboratories, Johnson Matthey & Co Limited

Stainless steel parts brazed with standard low-melting brazing alloys are susceptible to a specijic kind of corrosion by water or humid atmosphere, generally known as interface or crevice corrosion. One of the advantages of brazing alloys of the silver-copper-palladium type is that they will produce joints in stainless steel resistant to this form of failure.

Joints made in stainless steel with certain fast. Cases are known of stainless steel low-melting brazing alloys (of the silver- parts brazed with unsuitable materials falling copper-zinc-cadmium type) and exposed to apart after a week’s exposure to the action the action of water or humid atmosphere are of ordinary tap water. susceptible to corrosion of a rather unusual Another distinguishing feature of the corro- kind. sive attack of this kind, which is usually The attack in this case consists in the disso- referred to as interface or crevice corrosion, lution of a very thin steel layer along the joint is that neithcr the brazing alloy nor the stain- interface, as a result of which the bond be- less steel show any obvious symptoms of tween the steel and the brazing alloy is com- corrosion damagc. Although iron dissolved pletely destroyed. A failure of this kind is at the joint interface is precipitated as a loose illustrated in Fig. I, showing a laboratory red rust deposit outside the joint area, once stainless steel joint specimen in an advanced this corrosion product has been removed (by stage of corrosion. design or by accidcnt), there is nothing to Since the quantity of metal that has to be indicate that the separation of the brazing dissolved to produce this effect is extremely alloy from the steel was caused by corrosion. small, the rate of bond destruction is very In fact, the usual reaction of an observer

Fig. 1 A ferritic stainless steel (EN 60) specimen brazed with a 50Ag-lSCu- 16Zn-19Cd alloy and held for eight days in running tap water. The specimen was bent to reveal the extent of interface corrosion result- ing in the destruction of the bond between the brazing alloy and the steel

Platinum MetalsTev., 1969, 13, (2), 61-64 61 +-Brazing alloy

+Stainless steel

Fig. 2 Showing a blister formed in an austenitic steel (EN 58e) specimen brazed with a 54Ag-25Pd-21Cu alloy in dissociated ammonia under a flux rover and held fur 106 days in running tap ioater. ( x 50)

+-Sleel/solder interface +-Partially uxidised steel layer

Fig. 3 A magniJied view of a region of the specinten uf Fig. 2 shoat ing the presence uf a partially oxidised steel layer at a certain distance from the joint interface along a plane coinciding with the path of the crach whose formation led to blistering. ( % 550) faced with a failure of this kind is to con- brazing alloys with which joints resistant to clude that the joint fell apart because no bond interface corrosion can be made, each of these formation had actually taken place. materials has certain limitations. And so, The mechanism of interface corrosion, most alloys that are convenient to use from which now appears to be a much more com- the purely brazing point of view will produce plex phenomenon than first thought, has not corrosion-resistant joints but only when used yet been elucidated. However, extensive on stainless steel of the nickel-bearing austen- researches in this field have made it possible itic type. On the other hand, low temperature to classify various standard and specially brazing alloys that are suitable (from the developed brazing alloys in the order of their standpoint of interface corrosion resistance) usefulness for joining stainless steel when for joining both austenitic and ferritic stain- corrosion resistznce of the joints produced is less steels are often distinguished by some- an important consideration. what inferior flow characteristics. Finally, the It has been established for instance that, corrosion resistance of stainless steel parts although there are several low-melting silver brazed with low-melting alloys of this kind

Platinum MetalsTev., 1969, 13, (2) 62 may often be substantially affected by certain stainless steel parts is too low to make the changes in the composition of fluxes used steel surface wettable by the brazing alloy. during brazing. The usual remedy in such cases is to use The only known materials that combine the both a reducing atmosphere and a small freedom from the above limitations with quantity of a suitable brazing flux. However, many of the desirable properties of various it has been recently found that this practice silver solders are certain gold-base and may have unexpected and rather puzzling palladium-bearing alloys. The latter are consequences. exemplified by the 54Ag-z5Pd-z1Cu com- If an alloy is intrinsically capable of pro- position which is one of the Johnson Matthey ducing corrosion-resistant joints, it makes no noble metal brazing alloys known as Palla- difference whether the brazing is done in a braze 950; its special advantages as a filler reducing atmosphere or under a flux cover. material for joining stainless steel can be When, however, both a reducing atmosphere summarised as follows : and a flux are used in brazing with such an (i>Pallabraze 950 will produce interface- alloy (e.g. Pallabraze 950)~the resultant joints corrcsion resistant joints in both ferritic -while resistant to interface corrosion-may and austenitic stainless steels. be subject to failure of a different kind. (ii) The corrosion resistance of joints made When observed under laboratory condi- with this alloy is not affected by changes tions on specimens tested in running tap in the composition of the brazing flux, water, the failure consisted in the formation of especially by the presence of free boron which is sometimes added to brazing fluxes to improve their flow-promoting properties and stability at elevated temperatures. (iii) Joints made with Pallabraze 950 combine the resistance to interface corrosion with a relatively high strength at temperatures of up to 400°C. (iv) Stainless steel parts brazed with this alloy are not damaged by erosion due to excessive interalloying during brazing. (v) Due to its palladium content, Pallabraze 950 may be safely used when inter- granular penetration of steel by a molten brazing alloy might otherwise lead to brazing failures. (vi) Pallabraze 950 has excellent wetting and free-flowing characteristics and can be used both for brazing in air under a flux cover and for fluxless brazing in a reducing atmosphere. In this last connection a rather unusual Fig. 4 Showing the absence of any obvious differences in the structure of effect is worth reporting. joints brazed in an austenitic stainless It is sometimes found in industrial practice steel (ELN58e) with a 54Ag-25Pd-21cu that the oxide-reducing potential of a brazing ulloy: (top) in dissociated ammonia; (middle) under u flux cover in air, and furnace atmosphere which is sufficiently pure (bottom)in thepresence of both a reducing (dry) to preserve the initial bright finish of atmosphere and a brazing flux. ( Y 200)

Platinum MetalsTev., 1969, 13, (2) 63 a crack inside the joint. The crack was propa- the as-brazed condition revealed no differ- gated in the steel itself along a plane parallel ences between joints made with the same to, and at a distance of several microns from, alloy under a flux cover, in a reducing atmo- the joint interFace; it was often associated with sphere, and in the presence of both these the presence of a partially oxidised steel layer. fluxing media (Fig. 4). The crack plane approximately marked the Further studies will be necessary to estab- boundary of the zone into which palladium lish the precise conditions leading to joint and copper had diffused into the steel during failure of the kind described above and to brazing. The only external evidence of the ascertain that the effects observed were not development of this failure was the formation due to some extraneous, as yet unidentified of blisters in regions where the brazing alloy factors. Nevertheless, the evidence available coating was sufficiently thin. Photomicro- so far is sufficient tu advise against using a graphs of a specimen that had failed in this brazing flux in combination with a reducing way are rcproduced in Figs. 2 and 3. atmosphere in the joining of stainless steel It should be added in conclusion that parts that may be exposed to the influence of metallographic examination of specimens in water or humid atmosphere in service.

Thermocouples Under Neutron Bombardment CHANGES IN VOLTJME AND COMPOSITION Under severe neutron radiation thermo- mutation effects being confined to the couples are known to be unstable although platinum. The palladium content of palla- little data on the changes in composition which dium-platinum alloys is not greatly affected occur has hitherto been published. A recent although small quantities of mercury, gold, report by C. B. T. Braunton, D. N. Hall and cadmium, silver, iridium and rhodium are C. M. Ryall, of the Atomic Energy Research formed by transmutation. Molybdenum- Establishment, Harwell, (U.K.A.E.A., platinum alloys are considerably more stable A.E.R.E. - R5837, 1968)~now provides in- than rhodium-platinum alloys when irradiated formation which will greatly simplify the being comparable to ruthenium-platinum selection of thermocouple materials for ex- alloys in this respect. periments carried out under radiation at high Tungsten, rhenium and tantalum all suffer temperatures. Under such conditions lattice severely in a neutron flux. Some 40 per cent damage anneals out and changes of thermo- of the rhenium is lost after one year at a flux electric behaviour can be predicted from the of z x ~o'~n/cm~/sec;tungsten is rapidly compositional changes. These have been replaced by rhenium, rhenium by osmium, computed from differential equations which and tantalum by tungsten. The compositional were developed to describe the exponential changes in molybdenum are small. transmutation of isotopes, and curves defining Platinum alloys increase in volume under the composition of the various alloys at suc- neutron irradiation by amounts ranging up cessive stages of time and radiation are pro- to z per cent after one year at Io1%/cm2/sec. vided in this report. Tungsten, tantalum and rhenium decrease in Pure platinum when irradiated produces volume to a considerably greater extent. only gold and mercury, each reaching a A significant conclusion of this report is maximum concentration of rather less than that tungsten-rhenium thermocouples will I per cent after a year's exposure to a flux of suffer more damage, and are likely to be less ~o~~n/cm~/sec.Under similar conditions, stable than platinum-based thermocouples however, the rhodium content of rhodium- under similar conditions of neutron bombard- platinum alloys is almost- completely con- ment. It will be interesting to see whether sumed, to form palladium, mercury, gold and the results of the experimental programme iridium. now being carried out in mixed thermal and Ruthenium alloys are far more stable. The fast neutron fluxes, referred to in this report, ruthenium remains unchanged, all trans- confirm this prediction. A. S. D.

Platinum MetalsTev., 1969, 13, (2) 64