Are zinc plates bolts as good as stainless?

[xpress]

That's it really! Are zinc plated bolts as good as stainless!?

I've just had a read and they're nowhere near as good as stainless for corrosion resistance!

The bolt head says ty88.

[HarryMann]

If you don't damage the surface badly they are very good and to some extent self healing.
There are many types of steel and stainless particularly.
Some are quite weak being specified for serious corrosion environments. Others have good retention of strength at high temps but these tend to be specialist (expensive) alloys not always considered stainless (inconel etc)

The strongest (toughest) steels are usually unplated but corrosion treated e.g. black Cr-Moly.
Hence black impact sockets.

A lot of quality motor fasteners and many orig. VW ones were not zinc but various other finishes (some for low friction) and passivated (like anodized ally)
Simple hot dip galv doesn't give very fine tolerances but there are many types of zinc finish today.

Choose a bolt to equal the original spec. Most being 8.8 or 10.8.
There are some specials partic on the turbo diesel hotparts (4 costing £13 ea.) and others several £.

I wouldn't go around changing that many. The originals tend to be pretty good on VWs but there are exceptional situations... many have been there 25 years !

[clift_d]

In order of corrosion resistance, from least resistant to most resistant, generally:

zinc plated
zinc galvanised
non-austenic stainless steel (304)
austenic stainless steel (316)

Zinc coated steel will corrode eventually - how quickly will depend on environment etc. Austenic stainless steel such as grade 316 will last pretty much indefinitely in most situations.

The ty88 on the head refers to the grade of steel used in the bolt rather than the corrosion protection.

[AdrianC]

That's it really! Are zinc plated bolts as good as stainless!?

There's places you really don't want to use stainless fasteners. I forget the exact details as to why (I'm no metallurgist) but IIRC they should really only be used for cosmetic stuff.

The bolt head says ty88.

So 8.8 strength, not high-tensile.

[Plasticman]

used to get an awful lot of stuff cadmium plated, works very well subsea and splash zone, looks very good and does not build up un threads as some finishes

mm

[chapperss]

Yes high grade stainless bolts ie 316. they have all the tensile stength of cheese! stainless also has a tendency to "Bind" or pickup too! I tend to use copper slip to try and prevent this. like others have said.Your much better of with the correct grade mild steel bolt.

[clift_d]

As above, stainless steel doesn't have anything like the strength of high tensile steel, so for applications where strength is required you should use the correct grade of steel fixing - remember this is about the type of steel in the fixing and not the corrosion protection on the surface of it. For most other exposed applications stainless steel fixings would be your best bet.

[HarryMann]

8.8 equivalent is OK for many replacements but for suspension, brake engine mtg, some structural and many engine fastenings use the original VW part or match with a 10.8. Rarely is anything tougher required.
Two other points.. if a bolt has a plain part that can be important usually residing inside a casting or forging.. try not to replace with a fully threaded bolt.
If a large bolt has a fine thread be aware its for a reason and the bolt is doing a particular job.
Another quality often forgotten is toughness.. a function of core heat treatment & surface hardness.
Yup passivated cadmium plating is common on VW fasteners and used a lot in the a/c industry. Yellow & green tinges are typical.

[Plasticman]

and whilst be are on the subject.pedant head o9n fully threaded bolt = setscrew.machnine screw whatever, a plain shank of any length =a bolt
mm

[AdrianC]

and whilst be are on the subject.pedant head o9n fully threaded bolt = setscrew.machnine screw whatever, a plain shank of any length =a bolt
mm

^ and that's why I used a nice generic "fasteners"... <grin>

[tobydog]

In my experience of engineering, (cap) screws are used in tapped holes. Bolts are used with a nut.

Never differentiated with regard to thread length of a screw, specified for example M6x12 & M6x100 both as cap head screws.

316 (EN58J from memory) stainless steel is soft and cannot be heat treated for hardness, I wouldn't use it for anything that needed strength or toughness.

Mick.

[HarryMann]

No sorry there's a much bigger story than that Mick...
Whilst machine screws (fully threaded) are often used in that way, often for simplicity & versatility it's far from the full engineering story.

Practically first.. look at e.g. the bolts used to secure the disc brake calipers to the suspension upright lugs.
A very specific plain length to go through the casting (or forging) holes (also well toleranced) a very particular thread length and specification and some fine detailing (careful waisting) between the two sections.

Starter motor bolts ditto.
Drive shaft bolts very specific plain & threaded lengths. Dozens of engine bolts...

Theoretically... we have fretting considerations; shear between components (even though frictionally clamped, very often the function of a bolt is actually as a shear pin, and it just happens that a thread is used for retention rather than a circlip. You sometimes see a very reduced thread on a fat pin.. that's why; sometimes a combination of shear & tension;
corrosion might be considered (thread in job > air water hard to exclude c.f. plain fit); failure modes play a part sometimes ( that is how rather than if)..
; alignment issues best with a plain length bolt.

Today with rolled threadforms of very high quality some matters are perhaps less an issue and as always in engineering one can get away with murder for years until something bites back one day.
Obviously where things usually matter more than anywhere is aviation where weight is at a premiun but failures almost certainly cost lives. Additionally we have dynamic loads, vibration and maybe corrosion.
You can buy an AGS aircraft bolt with a plain length going up in about 2mm increments but all with the same std thread length (so many diameters or a set 12 mm on small bolts)
...they may be bolted up with a nut and washer or screw into a backing plate or the job itself... if so a coarser thread for a casting.

[HarryMann]

... and I suppose one could write a book on it too if not several

Cahpperess.. good point. Fretting and binding is bad with stainless, that's ture too. So copperslip or similar.

NB. I think A2 marking is used on s/s bolts that are nearly as strong a 8.8 ones.

Here is a very good pdf on these markings and strength of ss bolts..
Bolt markings and stainless Vs heat treated carbon steel bolts from volksbolts

[tobydog]

No sorry there's a much bigger story than that Mick...
Whilst machine screws (fully threaded) are often used in that way, often for simplicity & versatility it's far from the full engineering story.

Practically first.. look at e.g. the bolts used to secure the disc brake calipers to the suspension upright lugs.
A very specific plain length to go through the casting (or forging) holes (also well toleranced) a very particular thread length and specification and some fine detailing (careful waisting) between the two sections.

Starter motor bolts ditto.
Drive shaft bolts very specific plain & threaded lengths. Dozens of engine bolts...

Theoretically... we have fretting considerations; shear between components (even though frictionally clamped, very often the function of a bolt is actually as a shear pin, and it just happens that a thread is used for retention rather than a circlip. You sometimes see a very reduced thread on a fat pin.. that's why; sometimes a combination of shear & tension;
corrosion might be considered (thread in job > air water hard to exclude c.f. plain fit); failure modes play a part sometimes ( that is how rather than if)..
; alignment issues best with a plain length bolt.

Today with rolled threadforms of very high quality some matters are perhaps less an issue and as always in engineering one can get away with murder for years until something bites back one day.
Obviously where things usually matter more than anywhere is aviation where weight is at a premiun but failures almost certainly cost lives. Additionally we have dynamic loads, vibration and maybe corrosion.
You can buy an AGS aircraft bolt with a plain length going up in about 2mm increments but all with the same std thread length (so many diameters or a set 12 mm on small bolts)
...they may be bolted up with a nut and washer or screw into a backing plate or the job itself... if so a coarser thread for a casting.

Hi Harry,

Sorry, did I say that was the full story?

PS
I would only use zinc plated screws/bolts/nuts for ironmongery.

[HarryMann]

Nope Mick