Originally Posted by
Inferno
I'm just curious how you know only the early 96 versions are zinc plated under the e coat? This doesn't seem accurate in my experience. I've refinished a few sets, including a brand new set, and all have had zinc plating under the black e coat. All set were from the late 90s early 00s.
TLDR: Cost saving by Audi that resulted in later housings rusting out sooner
I've worked on the VW/Audi brand since 1978. I've probably worked on or inspected 400 B5's in the past 14yrs since purchasing my first B5. I started noticing that these housings and other multi piece/fabricated suspension components (i.e. rear subframes) on the 96 MY were rather impervious to corrosion when compared to later MY's. That's when I started looking closer and performed corrosion exposure tests. No this wasn't any standardized type test (ASTM B117) but it did prove that zinc was present.
Why are the early ones treated and the later ones not? Quite simply it's a product management decision called "cost reduction". Why add something that isn't necessary*to make a product through its life-cycle (warranty period). As a design and development engineer having worked for a Tier 1 supplier to major vehicle OEM's*(including the VAG group) this is commonplace. Removing a process step of this cost and volume saves $M's annually and the original purchaser isn't affected.
This happens on more parts than you would realize. The vehicle manufacturers*are businesses looking to maximize profits, not make 5th, 6th and later owners happy that their 20 yr old cars are maintenance free.
Zinc plating is applied after all fabrication and welding steps. As such it covers the entire part. This is either cold or hot. Some of these parts exhibit "runs" or streaks which are evident through the black paint top coat. This tells me the zinc was applied hot and not in a lessor phosphating process. I suspect the later parts (97 up) may have a Zinc Phosphate but I have no evidence to support that. On the contrary, the corrosion initiation points would indicated the part received nothing where weldments are present. Even the strictest vehicle industry specs these days are zinc-phos, KTL (e-coat), and then powder coat. Audi has a rust through warranty on the body, but that doesn't mean everything is covered.
The test is to chemically remove the paint and subject the part to moisture. I stripped a small section with paint remover and then left the part outside for several months. In my area we get 40 inches of rain annually. No red rust formed. The corrosion was slight but clearly it was white in color which represents Zinc Carbonate (ZnCO3). That has a rate that is 30 times slower in formation than iron oxide.
I doubt any aftermarket or remaining new OE parts have this. That's why these housings are quite unique. As these cars (specifically the B5/S4) continue to appreciate, it would seem that having OE parts like this installed will just add to their value.
Chris
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