Coating Behavior

Joined
Mar 26, 1999
Messages
8
I am interested in understanding more fully the behavior of various coatings used on blades. Can some others help me in my desire to collect some detailed information about the three general types of coatings: paint types, Teflon types and hard chrome. Does anyone have buddies in the arcane business of corrosion engineering?

Cobalt has given me some information and I would like to flesh out my understanding of diffusion barriers and actual observed behavior.

Let's get some reference material together and see if we can reach an understanding of what the characteristics of corrosion preventives are.

I posted a similar topic on Knifeforums http://www.knifeforums.com/ubb/Forum12/HTML/000702.html

BTW, please can someone tell me how to insert the wormhole stuff?

Thanks,

Noel


[This message has been edited by Noel (edited 28 March 1999).]
 
Noel, i'll keep this thread going with some info from my old school fading memories, for whatever it's worth, and in the hope that others who may have experience in this area will add to it.

Corrosion is a form of attack on metals and starts at the surface. It is an electrochemical process were the metal being attacked basically gives up an electron. This is oxidation. The metal were the oxidation takes place is called the Anode.

The following is a list of alloys that range from cathodic at the top to anodic at the bottom(that means most rust resistant at the top and most likely to rust and be ugly at the bottom):

Platinum(Would be an expensive knife)
Gold
Titanium
Silver
Nickel
Copper
Tin
Lead
Iron and steel
Aluminum
Zinc
Magnesium

At Iron corrosion starts becomming a concern.

Higher temperatures accelerate corrosion and usually anything that accelerates erosion will accelerate corrosion, like high winds, high fluid velocities, etc. In fact high temperatures can make alloys that are normally corrosion resistant, corrode, such as titanium. But the temperatures at which this happens are beyond what we can take anyway.

Stressed areas or risers such as cold worked bent(unstressrelieved) areas are anodic to non-stressed or non-coldworked areas. This means that if you stress relief job was no good, that is most likely were you will get rust to start first.

Typical metals corrode in a pitting form due to water droplets etc., on the surface. This pitting begins in any irregularity on the surface of the metal such as micro-indentations. Once the pitting or crevice corrosion begins it continues because the moisture has become stagnant within that area and allows an increased corrosion rate to oocur. The less irregularities there is the harder it is to begin the corrosion process. So an irregularity can be bead blasted finish so many of us like(I know I do). A polished surface is going to be much more corrosion resistant than a bead blasted finish because of the above reasons.

Anything that you use to coat a metal will increase it's corrosion resistance. Take a typical paint process on a car. You sand and acid etch to remove existing corrosion, then you primer and paint. The more layers of primer and paint you put on, the harder you make it for the path of moisture to get to the metal. Thats why so many old american cars rust. The old paints are lightly put on like your typical $39.93 paint job. A Rolls will have 25 to 30 layers of laquues paint on it, were a standard car may only have 5 to 10. The expensive cars have highly polished paints(buffed) and a lot of waxing. The wax fills the pores of the paint and does not allow loisture to get in as easily.

The same can be said for any steel. Chrome will provide a light measure of protection, as will a primer. But there are coatings out there, teflon and ceramic base that provide an almost encapsulating behavior within the meals surface, that does not allow moisture to penetrate for a long time. These coatings are classified in hours of resistance to salt water spray. So, for example, there is a well known coating that is reputed to be able to go 40,000 hours in salt water environment before corrosion can begin to take place. Of course, if you scratch the coating off, the rust will immediatelly begin at the exposed area and work it's way into the metal.

I'm tired of typing, so hope this made sense. This is all from memory so forgive any incorrect wording especially in reference to vehicles painting since I'm not a car painter by any means. this is all based on my learned theories and not practical experience, which can be different.
 
Noel, consider your sources of getting info. Would you go to a Dodge dealer to find out if their vehicles are good? I did not go to Mission knives and ask them if their Titanium was any good. Fortunatelly for me, I already knew what their alloy was, having worked with it in the aerospace industry. But I would have gone through other means.

There may just be a reason why the spec. ops people went to the TITANIUM MPK from mission knives. Could it be corrosion resistance? Definitelly was not abrassion resistance or edge holding, since the MD exceeds the Ti-MPK by far in those categories. The Ti-MPK was designed not to rust.

I found this after only a few minutes of looking on the net:

Pheonix Electroplating Co. who does several different forms of hard chrome plating;

"It is a common misconception that chrome plating provides corrosion protection. This is not the case, only slight protection is given. Hard Chrome is porous at the microscopic level, which can let moisture and chemicals through to attack the base material. If the item is to be used in a damp environment, or where water based inks are used on printing machines, a layer of Nickel plating is required, prior to hard chrome plating."


Here is another company's quote on corrosion and hard chrome:

"Refmet Ceramics Ltd, a leader in new and unique Advanced Surface Technologies, provides the long awaited bridge between conventional materials, and high-ceramics, overcoming the difficulties previously faced.

Alternative ceramic coatings based on flame and plasma deposited technologies, all result in porous structures, and even hard chrome plate has open grain boundaries, due to it's micro-cracked formation. These open migrant paths allow corrosive media to penetrate the coating and attack the substrate material, usually undetected due to the visual protection offered by the coating !."

And another quote for you from C-RAMIC company in Australia:

"Hard Chrome Plating
Although often used for wear and corrosion protection, has an inherent weakness.

It has a microcrack network, fissures and porosity sufficient to allow corrosive fluids to penetrate to the substrate/chrome interface, attacking it and causing debonding of the plating. "

Noel, there you go, it's all in front of you from three different sources all saying what I told you originally.

Three different sources(not including me of course because I'm nobody) telling you the same thing; "hard chrome is NOT NOT NOT a corrosion protective coating".

Now the next question is, does MD do something in addition to the hard chrome for resistance to rust. Like an additional ceramic coating?

Noel, I hope this has helped you, since I'm done beating on this dead horse.
 
I have some information to add as well.

TiALN and other titamium coatings deposited through Particle Vapor Deposition and salt bath also have grain boundaries, that is why the Black coating of Spyderco of a couple years ago did not prevent rust. The only PVD coating to prevent rust is Amorphous Diamond, but no one is using it on knives to my knowledge, and it is extremely expensive and you have to have minimum batches. Or so I have heard.

Now, none of these coatings, even without grain boundaries, do the job totally. For one reason common to all knives, the edge. This area is uncoated, then it is possible for rust to start there and creep under the coating. Fighting Knives reported that the final reason that the Navy stopped buying Mad Dog ATAKs is because rust was creeping under the coatings/going through the grain boundaries, now of course the party in question will deny this. From another source I am to understand that Mad Dog stopped blacking the blades so that one could see if rust had started under the coating and attend to it, of course none of this is verifiable except by parties who have moved on and don't care anymore or people who aren't going to denigrate their own product.

Now, please do not misconstrue this to be an attack(no pun intended) it is not, I did not bring MD up, the issue was already on the table.

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Check out the future possible Spyderco Worker 2000 www.bladeforums.com/ubb/Forum20/HTML/000224.html

One may want to keep an Eye out for my review of the Bob Kasper designed, Kevin Gentile modified AFCK and interview of Bob Kasper. Sorry about the wait, my review and interview should be online shortly, luckily Spark has graciously offered to do the pictures, look for it right here at BladeForums.com.

Marion David Poff ska Eye, one can msg me at mdpoff@hotmail.com

"A journey of a thousand miles begins but with a single step" Lao-Tzu
 
Marion, is the black diamond coating you mention, the one that spyderco tried unsuccesfully to put on the 440V Military and also the one that came on the Timberland Specwar which is supposed to give the surface a super high rockwell number?
 
Cobalt,

Thanks for placing all that information in your posts! It is not beating a dead horse for me because I really want to understand the various issues.

Regards,
Noel
 
Sorry, Noel, about the beating the dead horse comment. I was tired last night. I hope that the info. provided helps you. And don't let any of this stop you from getting the knife you like. Just remember that you will need to maintain it as you would any other carbon steel knife. This is good practice anyway. By the way, I am not just singling out hard chrome, but I don't care for coatings on knives in general because I prefer to see the rust develope. I don't like baked on epoxy coatings that you see on some of the CS and Ontario knives either. The good thing about those coatings is that they are easier to remove if you have to. These coatings also hold out moisture better than chrome, but they scratch off easier.

 
You are right about the suber rockwell, because as yo posted they form a kind of ceramic the titanium based PVD coatings.

But no one to my knowlwedge is using the amorphous diamond.

Most are either TiNAl or TiCN, I think.

MDP
 
Thanks to everyone for the information. I feel like my original questions have been answered.

I am amazed at the difficulty I seem to have in engaging in a debate over the 'net.

Cobalt, no worries about the dead horse comment, I just meant that I was not being argumentative; I was indeed interested in the information.

Thanks,

Noel
 
ask away. i rep 4 a co. thast does ti coat
i,ve already seen several misconceptions promlugated
harley

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ask away. i rep 4 a co. thast does ti coat
i,ve already seen several misconceptions promlugated
harley

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Harvey, if you know considerably about coatings then answer the following questions, if possible:

1. Are there any coatings as abrasion resistant as chrome that offer more corrosion resistance and stay bonded as well?

2. Are there any coatings or any new chroming process involving other materials that are more corrosion resistant than chrome?

3. I gather that you have experience with titanium coatings. How do these differ from chrome, ceramic and teflon base coatings?

 
Harvey do u mean me?? harley , if you know considerably about coatings then answer the following questions, if possible:

1. Are there any coatings as abrasion resistant as chrome that offer more corrosion resistance and stay bonded as well?
all the ti coatings have more abrasive resistance and should bond better

2. Are there any coatings or any new chroming process involving other materials that are more corrosion resistant than chrome?
chromeium carbide applied by vapor deposition
3. I gather that you have experience with titanium coatings. How do these differ from chrome, ceramic and teflon base coatings?
these r all thick coatings
a coatings rust resistance is determined by its porosity
ck out tigolds web sight for more answers
harley
it was a stick
possum
 
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