FOCUS ON MATERIALS
Figure 3 : A bipolar plate features a complex pattern of channels formed into its surface .
Outokumpu is currently working with several fuel cell manufacturers to help it perfect the pressing and welding processes that will facilitate the mass production of bipolar plates
the current ‘ state-of the-art ’ material . This is a molybdenum-alloyed , austenitic grade that offers the ideal combination of electrical conductivity , high corrosion resistance , excellent formability and high strength for the manufacture of bipolar plates . In addition , the material is an integral part of international standardisation and is available worldwide .
Stainless steel offers a high level of natural corrosion resistance , thanks to the passive oxide layer that forms on its surface . However , manufacturers still add a coating to the side of the bipolar plate that is in contact with the electrolyte membrane . This coating , which is only about 5 microns thick , plays a dual role . It provides very high electrical conductivity and also protects the base layer against the very corrosive electrolyte . The need for this coating also applies to rival metals such as titanium .
An area where EN 1.4404 excels for the pressing of bipolar plates is its excellent formability , with an elongation A80 ≥ 45 percent according to the standard . However , for actual deliveries this is mostly on a level of A80 ≈ 60 per cent . This allows precise and repeatable pressing of flow channels with sharply defined shapes and tight radii into the surface of very thin bipolar plates .
As an indication of the exacting nature of the pressing process , the deepest channels in typical bipolar plates are just 0.1mm deep . The current state of the art has been set at 0.075mm , and we are already looking at new designs with channels of just 0.05mm .
Working with fuel cell manufacturers
Outokumpu is currently working with several fuel cell manufacturers to help it perfect the pressing and welding processes that will facilitate the mass production of bipolar plates . The aim is to verify that the stainless-steel foil provided will work with their specific forming machinery .
Normally , for bipolar plates , our precision strip mill in Dahlerbrück , Germany , will supply uncoated stainless steel foil in thicknesses of 0.1mm down to 0.075mm . It is feasible to produce a foil of 0.05mm . Beyond that , considerations of strength and stiffness probably mean that a further reduction in thickness is not desirable .
The key to success is not only in producing the final thin foil . Careful attention to the properties and treatment of the stainless steel is essential as it passes through the intermediate stages . This ensures that it will exhibit the best forming behaviour for the manufacture of the bipolar plates . The intermediate treatment steps are an area on which Outokumpu places an intense focus to deliver the optimum pre-material for fuel cell manufacturing .
Forming the hydrogen economy
Fuel cells are expected to make an important contribution to decarbonising transport . It is therefore desirable for them to be manufactured from recyclable materials , where possible . Stainless steel is a superb material for sustainable solutions as it is long-lasting
The Supra range
Outokumpu ’ s Supra range contains austenitic and ferritic stainless steels grades for medium to highly corrosive environments in PRE range 22-26 . The most widely used molybdenum alloyed austenitic stainless grade , Supra 316L / 4404 ( EN 1.4404 ), defines the overall landscape of the Supra range .
The range is complemented by a modern stabilised molybdenum alloyed nickel-free 18 per cent chromium ferritic stainless-steel alternative , Supra 444 / 4521 ( EN 1.4521 ). The latest Supra range addition is the austenitic Supra 316plus / 4420 ( EN 1.4420 ) with higher corrosion resistance and strength , compared to 316L .
“ Modern Supra range stabilised molybdenum alloyed ferritic stainless steel , Supra 444 / 4521 , has excellent corrosion resistance , welding and deep-drawing properties . Nickel-free ferritic stainless steels grades have excellent resistances to chlorideinduced stress-corrosion cracking . Ferritics exhibit good heat transfer properties that can be utilised in applications where uniform and rapid heat transfer is preferred . Good heat transfer with lower thermal expansion eases weldability via smaller weld distortion and buckling behaviour than austenitics . Ferritics also have similar machinability as carbon streels , due to low work hardening characteristics ,” added Outokumpu .
and 100 per cent recyclable at the end of life .
Outokumpu ’ s carbon footprint is less than 30 per cent of the global industry average for stainless steel . This is mainly because it uses its own ferrochrome , has a high recycled content of more than 90 per cent in production and uses low-carbon electricity .
As the world makes the transition to decarbonised transport , developing a hydrogen economy with the widespread use of fuel cells will be crucial . For this to happen , manufacturers will need to take fuel cell technology that is already well proven in low-volume applications and adapt it to suit the needs of the mass market .
The challenge will be reducing the size and weight of fuel cell stacks as well as making them available at a competitive price . This is where stainless steel offers major potential . It can help manufacturers to design durable and high-performance bipolar plates that can be pressed into shape in very high-volume production processes .
30 | sheetmetalplus . com | ISMR February 2022