Black phosphate head bolts/studs, rusting

I'm switching to aluminum heads on my chevy small block and I want the heads unpainted.

Consequently the head bolts, which are only black phosphate treated, will rust?

Is there a way to keep them from not rusting? Or brand that is better treated than others?

Since they are high strength bolts I suppose most surface plating procedures are off the table?

 

Zinc phosphate reduces the chance of any galvanic corrosion between aluminum heads/parts and the high alloy steel bolts, so they're perfect for aluminum heads. Zinc is a good barrier between steel and aluminum. I don't see any chance of them rusting under the hood, especially with a light wipe down with some light oil. What brand are the head bolts? Plating the bolts seems excessive! And plating the shoulders would screw with torque values. I stick to ARP bolts, if I can get them for a particular engine.

 

"Zinc phosphate reduces the chance of any galvanic corrosion between aluminum heads/parts and the high alloy steel bolts, so they're perfect for aluminum heads. Zinc is a good barrier between steel and aluminum. I don't see any chance of them rusting under the hood, especially with a light wipe down with some light oil. What brand are the head bolts? Plating the bolts seems excessive! And plating the shoulders would screw with torque values. I stick to ARP bolts, if I can get them for a particular engine."

But "Black Phosphate" is not zinc phosphate. Zinc Phosphate is phosphate coating OVER zinc. Black phosphate by itself was never meant for anything other than mild environments and moisture. It is truly a "storage protectant" or coating only.

- EM

 

Black Phosphate is not zinc. It is iron phosphate, and is a crystalline surface. Most have an oil dip, where the crystal structure holds the oil. That is what provides corrosion protection. By itself iron phosphate is almost no benefit. Phosphate is actually a great paint substrate, just make sure to remove any oil that is probably on the bolts. The same crystal structure is great for paint adhesion.

Zinc plated parts typically have chromate coating over the zinc. Chromate can be clear (actually very slight blue tint), yellow/gold, olive drab, or black. Black chromate is expensive, at least in comparison to the others. I believe it is silver nitrate(??) that makes the black color.

Heat treated bolts can be plated, but you must do a hydrogen embrittlement relief bake. Any plater worth using should require this for any fasteners, but must be done for any heat treated Grade 8 or stronger bolts.

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Brush paint the heads after you put it together.
Easy peasy and probably cost you 3.00

 

I've had good luck coating bolts by applying a quality rattle can paint in 3 light coats giving it time to cure in between then baking the paint on in an oven or toater oven at 350. a garage sale toaster oven is worth having around.

 

Bring this back up for clarification. What is "Parkerization" which I was told is the process for a black coating on some fender bolts I have for a '56 Ford.

 

Parkerizing is the trademarked name for a phosphating process. Same thing.

 

Manganese and/or iron phosphate is dark grey to black with large crystals. Zinc phosphate is light to medium grey with fine crystals. Often, all 3 are used in the same phosphating bath.

 

I know this is not a motorcycle site, but on my 48 HD, I have pakerized bolts all over touching aluminum and I do not see the typical trouble with dissimilar metal. I think 316 SS is a big problem in contact with aluminum. Of course there are numerous aluminum alloys with different physical properties.