Main.TrailIndexPage | Chassis and Body | Cadmium plating and other coating details

Cadmium Plating and Coating

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  • Cadmium plating


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Cadmium plating

I have now decided to bite the bullet and use new hardware or replate components in the engine compartment. My question is: are most or even all of the screws in the engine compartment gold cadmium plated? Mine are worn and sometimes I can't tell if they had been plated or not. For instance:

  1. I'm not sure about the alternator adjusting bolt (or stay).
  2. The same for the cast iron levers that are connected to the various throttle linkages running across the top of the engine. (I know that the linkages themselves were cad plated. I'm just not sure about the levers with the ball connectors attached at the ends.)
  3. How about the hose clamps for the coolant overflow tank and power steering reservoir? (I could swear mine hadn't been plated)
  4. Does anyone know whether the various smaller coverings attached to the windshield frame that are not chrome plated have any plating on them or are they simply made of stainless steel. Could they be silver cadmium plated or have a galvanized plating? I'm referring to part numbers; 113 688 09 39, 113 688 07 39 and -0839 and 113 688 1039. Thanks for any help with these last restoration items.

Frank replies: re 1 and 2: Yellow cad. Re 3: The bottom clamps were cotter-pin type (all of which were white cad). Small vent line clamps white cad. Re: 4. I think they are stainless.

Frank, I'm not quite clear on which bottom clamps you are referring to on the power steering reservoir?

Frank replies: I thought you were asking about clamps on the hoses that go in/out of the water tank (I think you called it an "overflow" tank - maybe I misunderstood you. Which clamps were you referring to?

I was referring to the large hose clamps that hold the coolant overflow tank in place as well as the power steering fluid reservoir. I have received an excellent picture of a brand new 280sl engine bay from Achim (thank you) that seems to confirm that these clamps were white or silver cad plated? I can't quite make out whether the brake lines are yellow or white cadmium , though? I also realized that there are several more parts that I need to take off the engine which should be cad plated, but are no longer on mine.

Are you talking about the clamp that holds the reservoir or those that hold the power steering hoses? What are the small vent line clamps?

Frank replies: The hoses at the PS reservoir took a special clamp of the same type as used on the short fat hose at the bottom of the thermostat housing. They are NLA.

And how about the clamps that are all over the fuel injection lines?

Frank replies: hopefully, you are not talking about hose clamps for use on injection lines, because there should be none on an M130E engine. The clamps that hold the pipes in place are still available in the original yellow cad from your MB dealer.

All the linkage for the fuel injection pump should be cad plated then yellow chromated. The exceptions are anything that is aluminum. A rule of thumb is if its aluminum, it is not plated. If it's steel, it always has an anti-corrosion finish on it. Usually all clamps, linkages, brackets etc. are yellow cad plated. The other items like booster, master cylinder, resevoir for power steering etc are painted black.

Hoping someone knows about underhood zinc plating: I've seen a 113 or two with replated parts (injection pipes, control shaft) under the hood; the plating is yellowish in color. But when I look at my '69 280SL (Euro), there is no evidence that those parts were ever zinc plated. There is not a trace of zinc anywhere. I guess it could all have disappeared over the past 33 years, but it seems unlikely that it would leave no trace at all. Were the injection pipes and control shaft zinc plated on all 113s? Could there have been a different kind of plating, or no plating on some cars? Thanks in advance for any info!

Caswell Plating sells a "copy cad" kit that allows you to replicate this finish. I have this kit and have used it extensively for my car. The yellow finish is yellow chromate and is achieved by first plating a part in zinc (through electrolysis) and then "etching" the freshly painted zinc with yellow chromate. So the zinc is plating and the yellow is etching or staining.

I've never seen any other cad-plating aside from what comes on the new replacement parts. Even back in 73, when these cars were still fairly fresh, it still looked like the stuff on new parts.

Dan Caron: It's not cad plating. It's zinc dichromate. Cad plating is white -ish in colour. Zinc dichromate is a gold with sometimes a bit of green in it. The part is first zinc plated and then dipped into a chromate wash or solution. Strength and time of immersion will determine the colour to some extent.

Bernt Damm: My original 230 SL has no such yellow plating either. Zinc yes but yellow no. Especially the injector pump parts and pipes are certainly not yellow. Most of the plating is silver in colour. Perhaps they changed the plating, depending on their suppliers? If I had to redo any of it, I would go for cadmium plating because it seems to last better. That is, if I can still find someone to do that. Anyone else have non yellow plating?

Tom Sargeant: As I understand the process, the yellow chromate (dichromate) is "sacrificial" and fades over time such that the original zinc finish remains. Zinc is also sacrificial, and when it oxidizes away, rust begins. The silver gray color on the injection lines and other parts that were originally yellowish in color have now oxidized. I could be wrong, but this is how I understand how the color changes over time.

Bernt: I am no expert either but I do have a car port structure that has 4 yellow zinc plated poles on it. We are near the coast and things do rust. The poles were put up in 1987 and they have been there ever since. The yellow is still there. It is correct to say that the zinc is sacrificial. In fact, I have just been to a chemical engineer who mixed some zinc plating solution for me. He said that the zinc protects itself against corrosion by forming a layer of 'white rust'. However, this gets prostponed further by the zinc being passivated after the plating is completed. This kinda closes off the pores in the plating and the plating should last a very long time. I can also tell you that I had a lot of brackets, nuts and bolts zinc plated and yellow passivated not too long ago. It was for my 220Sb finny. This looked fine but I noticed then already that I could wipe the yellow off if I rubbed hard. Needless to say, the plaiting has since gone dull and is forming white corrosion. They also lost some of my small clips in the process. That is the reason why I want to do it myself now. Anyway, I am fairly sure that yellow parts remain yellow because on my '64 220Sb, they were faded and grubby but still definitely yellow. Same on my poles. Do you think the 'original 280SL' picture mentioned earlier was taken in 1968 or recenly? If recently, I doubt that that plating is original. Any other opinions on the plating?

Is that "zinc" or cadmium-plated? I thought the latter (hence, the yellow-ish cast to it).

Achim: It is actually the first, zink and zink with a variable amount of copper. If you increase the amount of copper to (I think to remember) 40 - 60 %, you will get brass.

Dan Caron: No, it's zinc in all cases. Zinc is probably cheaper and easier to apply. Not sure really. It's the chromate wash that gives it the yellow colour.

Tom Hanson: Technically speaking, yes, you are correct. "Cad" has become sort of a nickname or slang term used popularly here by us "Yankees". Thanks for the tip. What would happen if you put a clear-coat on these parts?

Dan Caron: nothing particularly useful. Works for a while , turns yellow and then flakes off in most cases. An aluminum or silver coloured paint might be more useful.

Achim: You are pretty correct about the zink plating. It stays yellow for a long time but it looses the color a bit over time, especially after almost 40 years. Furthermore Pete Lesler once mentioned that earlier cars (ours) had less yellow chromating than later cars. If you unscrew lines and clamps etc. you will find more yellowish chromating remaining underneath where not so much rust was/is.

Tom Colitt: I just wanted to chime in a little too, on the zinc di chromate discussion. I am quite certain that those of you who believe that some of their injection parts were not plated like this, in fact were. I have seen many cars in various stages of aging and one can clearly see the evolution from the originally quite golden blotchy look to the dull Grey look after 20 to 40 years. Even newly re-plated or MB replacement parts deteriorate a little bit fairly quickly.

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