Keith
Thanks for this comprehensive explanation. However, there is one
possible omission: the role of halides as flux activators, even when
present in minute quantities. Such organic halides or their inorganic
decomposition products will dissolve the refractory oxide layer on all
stainless steels like gangbusters, leaving it exposed to the actions you
adequately describe.
Brian
[log in to unmask] wrote:
> Harvey,
>
> I am surprised that it has taken so long for the news to get out about
> the aggressiveness of the tin-silver-copper (SAC) alloys towards the
> stainless steels commonly used for the manufacture of wave solder
> pots. This phenomenon has been known for several years but perhaps
> the advocates of SAC alloys for wave soldering have not been keen to
> see it get too much publicity.
>
> This phenomenon is one reason why many companies have opted for the
> tin-copper which (as pointed out by Gebhard Neifer in another
> contribution to the forum), when turned into a useable solder with an
> addition of nickel, seems to be much less aggressive towards solder pot
> materials. I will not claim that none of the 400 or so machines
> currently running the SnCuNi have suffered any erosion but they have
> been relatively few and those that have occurred seem to be due to a
> different mechanism. Many of these machines, which have only plain
> uncoated stainless steel pots have been running for up to four years
> without failure due to erosion.
>
> As you know stainless steel is "stainless" because of the very stable
> oxide film that forms on the surface. Once that oxide film is
> penetrated then any high tin alloy, including "good ol' 63/37", will
> erode the underlying ferrous alloy. The degree of aggressiveness is
> related primarily to the tin content so the high-tin lead-free alloys
> are more aggressive in dissolving stainless steel (after the oxide film
> has been penetrated) than 63/37 where the tin is diluted about one third
> with lead which is inert towards the steel. The first stage of the
> erosion process is the formation of an iron-tin intermetallic, FeSn2.
>
> The protective oxide film can be broken down in several ways.
>
> One is by mechanical attrition. Tin oxide, which is always formed to
> some extent on solder, is a very abrasive material (it is used for
> barrel polishing gemstones) and in parts of the pump and wave forming
> equipment where solder is flowing at high speed past a surface the
> protective oxide on the stainless steel can be rubbed off exposing the
> underlying metal to the molten solder and starting an erosion
> process. Where there is cavitation occurring, e.g. on the leading
> edges of a pump impellor, the effect is exacerbated. This effect
> occurs to some extent with all the solder alloys, including 63/37 where
> erosion of pumps, manifolds and wave formers is not entirely unknown.
>
> However, a study that I reported at APEX 2003 this year found that there
> seems to be an additional effect occurring with the SAC alloys,
> particularly when they contain an additive commonly used to control the
> severe drossing that tends to occurs with these alloys. It seems that
> the SAC alloys can break though the protective oxide chemically even
> under static conditions and wet the underlying solder. Once the
> underlying steel has been wetted erosion will proceed. Further work
> needs to be done to confirm the exact mechanism by which the SAC alloys
> penetrate the oxide film but it is suspected that the fluxing action of
> the anti-drossing additive and the silver work together to break down
> the oxide film on the stainless steel very quickly. And, experience in
> actual wave soldering machines is that, once penetrated the SAC alloys
> seem to dissolve the underlying stainless steel more quickly than the
> SnCuNi alloy.
>
> All wave solder machine manufacturers are now aware of this problem and
> offer various coatings to protect the stainless steel and I think it is
> certainly worth taking advantage of this extra level of protection when
> buying a new wave soldering machine or a new solder bath. However,
> those who need to change to lead-free before they can afford to purchase
> of a new solder pot should be aware that lead-free solders vary in their
> aggressiveness towards stainless steel depending on their composition.
>
> When changing a tin-lead pot to lead-free all the surfaces of the pot,
> pump and wave forming assembly should be checked for evidence of wetting
> by 63/37 before refilling. Solder should be stripped from any wetted
> areas, the surface rubbed back to bare stainless steel and the oxide
> film allowed to reform before filling with solder.
>
> This issue is prompting people to check more carefully the actual grade
> of stainless steel used in the construction of their solder pot. It
> is not appropriate to go too deeply into the metallurgy of stainless
> steel in this forum but the "304" type is more likely to suffer erosion
> than that "316" type. That is because the 304 is more likely to have
> some ferrite present in the microstructure and that does not have such a
> tough oxide film as the austenite of a true stainless steel. And, as
> you suggest, the problem can be exacerbated by poor welding techniques
> that can leave more ferrite in the microstructure.
>
> In summary I agree that this is an issue that the industry should be
> aware of and manage appropriately.
>
> Keith Sweatman
> Nihon Superior Co., Ltd
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