Author Topic: Bimetal corrosion?  (Read 230 times)

Carpenter84

Bimetal corrosion?
« on: October 03, 2020, 10:39:20 PM »
im working with a large quantity of aluminum, in order to machine perfect alignment i am utilizing dowel pins in a few locations. the customer just brought up the concern of bimetal corrosion between the aluminum and the steel dowel pins. these parts will be in a high temperature fluctuation atmosphere.
while i do understand aluminum can corrode and can be a reactive metal... but what kind of risk are we talking here? should i use a different type of dowel pin? the customer asked if i was using aluminum dowel pins... im not even sure my supplier can get them, id have to find out... is it actually worth trying to obtain?
Shawn

First 9x42 column mill,
Enterprise 10x28 lathe,
Ko Lee 6x12 surface grinder,
Airco dip/stick 160 welder,
Fully stocked wood shop.

RayH

Re: Bimetal corrosion?
« Reply #1 on: October 04, 2020, 02:14:04 AM »
The question is a design/engineering question. Are you being paid for that? My point is, the customer (design responsibility) should tell you how to build it.
That being said, . . .
I have no idea what "high temperature fluctuation" means. Specify Minimum degree and maximum degree range. If we're going to get serious, what is the rate of temperature change?
The simple answer is, . . .
Use 416 stainless steel pins (per MS16555).
We used to install them into 6061 (military and space equipment) without reservation.
Raw 6061 was always Chromate Conversion Coated per MIL-DTL-5541, Type 1, Class 3. For space applications, usually gold plated the 6061.
However, the pins were always press fit, so I think the chromate coating (soft) would be wiped out by the interference fit so the 416 and 6061 ended up in intimate contact anyway. An alternative to press fit is to Loctite them in which may isolate the two metals (talk to Loctite Tech Rep).
« Last Edit: October 04, 2020, 04:00:28 AM by RayH »
Ray

Carpenter84

Re: Bimetal corrosion?
« Reply #2 on: October 04, 2020, 05:57:31 AM »
No, I wasnít paid for engineering.
Temp swing would be room temp, to 482*F.
I could oversize the hole and loctite, but Iím not sure thereís time now to order. But stainless dowel pins would be on the shelf at the fastener wholesaler.
Shawn

First 9x42 column mill,
Enterprise 10x28 lathe,
Ko Lee 6x12 surface grinder,
Airco dip/stick 160 welder,
Fully stocked wood shop.

4GSR

Re: Bimetal corrosion?
« Reply #3 on: October 04, 2020, 07:23:05 AM »
You know, 482 F is enough to change the properties of 6061 aluminum along with theromo expansion.  That can expand enough to make those dowel pins loose in the assembly.  Not much you can do about that, more press fit is going to over stress the aluminum around the hole and still have the same problem when heated.

If it was me, I would give those dowel pins a quarter to half thousandth clearance, not press fit, a get a can of that Never-Seigh for aluminum, and apply this to the dowel pins and the holes. insert the pins, pull, reapply, then assemble.  May have to get some Q-tips to use to apply the Never-Seigh in the holes.  This stuff will help prevent galvanic corrosion at high temperatures, where Loctite will fail at around 300 F. 
Ken

Carpenter84

Re: Bimetal corrosion?
« Reply #4 on: October 04, 2020, 07:48:21 AM »
the dowel pins are not press fit as is. i can push them in with my thumb. I am using a 3/8" reamer with cutting oil, and the dowel pins are measuring .0002" over, I do not have the capabilities to accurately measure the bore... this is edging beyond my measuring capabilities. in any event, the pins are not pressed in.

i presented the stainless option to the customer, he advised me the reaction would be worse at temperature and opted for the alloy steel pins. I suggested the aluminum pins would be too weak for this application - granted, the pins are merely being used for assembly and disassembly while i build, and alignment while the sections are being welded together. also, loctite would flash off during welding.

the customer clarified the max temp as 200*C, with the possibility of reaching 250*C, but not a regular occurrence.
nothing is ideal. the entire project has been very challenging to engineer.

the product currently in use is a large aluminum frame, 114" x 54", the frame being made from 2" extruded aluminum channel, mitered at the corners and fails constantly. i have actually made several small pieces for the customer previously in an attempt to band-aid the problem. the current project is a complete replacement, with solid material, and instead of being joined in the corners, the frame is joined about 8" beyond the corners (a C shape joined with bar stock on the long sides) leaving the corners as solid material. the joints need to be welded together and the transition from one section to the next be machined smooth.
and yes, the frame is repeatedly heated and cooled.

i suspect his current product fails in the corners due to the heat and expansion. I do not know how my product will fare in the same environment, I have come up with the best ideas my limited experience can muster.
Shawn

First 9x42 column mill,
Enterprise 10x28 lathe,
Ko Lee 6x12 surface grinder,
Airco dip/stick 160 welder,
Fully stocked wood shop.