Saucepan - corrosion of aluminium base piece 1

[SteveyG]

This is a interesting case study i think, for anyone interested in corrosion/failure analysis. The question is - what on earth has the user done to this pan!?
If you look at the pics (the white bit on the base is just where i blanked out the manufacturer)- this is a standard encapsulated base/sandwich base saucepan with a copper base sheet which is either impact bonded or brazed (in this case i believe impact bonded)onto the aluminium base piece (core), which is bonded to the bottom of the stainless steel pan body.
So its a sandwich of copper base sheet, aluminium core and the stainless pan body.
The copper sheet wraps around and up the sides of the pan body a bit of course.
Now this arrangement is not necessarily watertight (in a dishwasher etc) and some small amount of white aluminium corrosion product can sometimes 'ooze' out - however I have never seen anything like this.
I am not sure but it certainly looks like the aluminium has very seriously corroded, the corrosion product has pushed at and deformed the copper sheet and also peeled it away from the pan body. The copper sheet is raised up around the entire circumference and stands up about 4mm.

Theres a crevice and galvanic corrosion possibilities in this design, but I never saw an example like this, so I'd welcome any comments as to what might have happened.

am I right in thinking its 'corrosion product pressure' that has deformed the copper? theres a bit of the copper has a 'new' clean surface and some green corrosion product so perhaps the user has been cleaning the copper in acid or strong alkali? and got this substance under the copper and in prolonged contact with the aluminium. then its in use, gets hot, gets washed, starts to get more and more water in..and it goes from bad to worse??

I dont think its steam pressure in use because i would have thought that would blow its way out in one area, not the full circumference, and anyway the entire circumference is filled with the white corrosion product.


Thanks

 

[SteveyG]

oh only one pic was attached - heres another - cross-section of the
pan base

 

[SteveyG]

and another pic showing the copper base circumference deformation

 

[SteveyG]

pic of 'clean' copper surface and green corrosion product

 

[SteveyG]

last pic - wire brushed off the white corrosion product - the aluminium is certainly extensively corroded, so assume the source of all or most of the 'white stuff'

 

[EdStainless]

Prob sat in low pH solution (dishwasher soap) and galvanic corrosion took over.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[EdStainless]

Sorry, I meant high pH. one of those days

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[swall]

Some dishwasher detergent formulations, mainly the cheap brands, contain sodium chloride.

 

[SteveyG]

yeah, alkali detergent and salt in the detergent, great combo for getting trapped in a crevice with dissimilar metals in contact !

then heat applied in use speeding up any chemical attack perhaps, maybe concentration of any corrosive stuff by evaporation in heating too, then back in dishwasher to get water trapped inside, then put it somewhere and don't use it for a while with water trapped inside....theres so much to go wrong, its amazing actually it doesn't happen more often.

maybe in this isolated case - something used to clean the copper got trapped , and/or a especially 'leaky' copper sheet.......??

Materials technologist/metallurgist

 

[IRstuff]

I'd have to argue that something else occurred. Obviously, the something got between the aluminum and BOTH the inner and outer layers of metals. I just can't see getting that much damage from soaking, or washing, in whatever environments in any sort of reasonable time span, unless the user left it in some weird chemical bath for months or years.

It might make more sense that the pot was somehow mechanically damaged, perhaps by excessive heat, to the point where the copper and aluminum delaminated from the pot to start with. This would then allow free access for fluids to enter the interstitial spaces and get a good reaction going.

Perhaps, the user soaked the pot in CLR to remove the scorch marks on the copper, not realizing that CLR would work great at gobbling the aluminum, particularly any formulation that still has phosphoric acid.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[berkshire]

Is there a possibility the user used lye ( caustic soda ) to clean the pan. It would have no effect on the stainless or the copper, but would raise hell with the aluminum.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[SteveyG]

Ok thx guys.

There did not seem to be any signs of overheating - usually the inside of the pan will show evidence of this, but then again the copper could still have got hot enough to be damaged as suggested.

Having tried to get more info - I still don't know how long the pan was in use, it appears from the same user there was a complaint a while ago regarding tarnishing of the copper (which is normal obviously) so this backs up the theory that the user cleaned the copper in something that may have got trapped inside.

i dont think by design that this kind of base can be guaranteed to be watertight, but maybe this was an especially 'leaky' one, or did get damaged.

regarding the copper and aluminium delaminating, these were 'joined' by impact bonding - a process i know very little about at the moment - and the join was very good elsewhere, but it could be that round the circumference part it was not good to start with (or was subsequently damaged as suggested).

don't even know if the user continued using it as the corrosion product was building up and the copper bulging out......? certainly would have been useless on a flat hob, but on a gas hob would be OK i guess.

oddly the user seems unwilling to say exactly what she did - since a replacement pan has already been given and theres no injury claim etc, theres no reason not to, unless she thinks the pans gonna be taken back off her ! I'd like to know exactly what she did, would save much postulating of theories...



Materials technologist/metallurgist

 

[SteveyG]

caustic soda, yeah good idea - common household stuff.

Ok my theory - she leaves it sat in caustic overnight to clean the copper - the copper has blackened in use or perhaps overheated.

(the pan is a bit 'leaky' already, or got more 'leaky' due to overheating.any delamination of copper/aluminium would create a terrific crevice and bi-metallic corrosion of the aluminium...)

caustic is trapped inside - pan used so gets hot obviously - speeds up the caustic reaction - also concentration by evaporation so a very nasty corrosive chemical - user uses it and dishwashes afterwards - so keeps getting more water inside then sat for a day or 2 moist like that in between uses, but the caustic never gets washed out completely, then used and heat/concentration effect again, more and more corrosion product builds up -pushing at the copper base and making it more and more 'leaky'..snowballing until the user finally realises the pan is a complete write off !



Materials technologist/metallurgist

 

[berkshire]

unless more caustic gets into the joint , at some stage the reaction has to stop. When the caustic soda has all reacted with the Aluminum and formed Aluminum Hydroxide. Unless of course it is being replenished.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[blacksmith37]

Detergents normally contain TSP (tri sodium phosphate), called
'builder"; the EPA is restricting its use.
But ,with copper in contact with aluminum (and moisture) and an electrolyte, the corrosion won't stop untill the aluminum is gone.

 

[SteveyG]

maybe you guys would regard it as obvious, but I was amazed by the deformation of the copper. its pretty thin sheet and its relatively soft copper but still amazing to me

of course this was just a pan and it aint going to kill anyone, but
it really brought home the 'power' of corrosion product build up and the damage it can do.

Materials technologist/metallurgist