The electrolyte fills the pits and prevents the oxygen from passivating the active metal so the problem gets even worse. This difference in relative areas accelerates the corrosion, causing the pits to penetrate deeper. Pit type corrosion is therefore simple galvanic corrosion, occuring as the small active area is being attacked by the large passivated area. Referring to the galvanic chart you'll note that passivated 316 stainless steel is located nine lines from the bottom and active 316 stainless steel is located thirteen lines from the top. As stated earlier, halogens will penetrate passivated stainless steel.

It occurs when the corrosive environment penetrates the passivated film in only a few areas as opposed to the overall surface. Pitting is an accelerated form of chemical attack in which the rate of corrosion is greater in some areas than others. This is one of the reasons we see corrosion under o-rings, and Teflon wedges. Shaft movement that causes fretting of the 316 stainless steel rubs off the passivated layer and exposes the active stainless to the electrolyte until the metal part becomes passivated once more. Many metal seal components are isolated from each other by the use of rubber o-rings or similar materials and designs. A large cross section piece will not be affected as much as a smaller one. Any stray electrical currents in the electrolyte will increase the corrosion also. The higher the temperature, the faster it happens. The temperature and concentration of the electrolyte. The distance separating the metals on the galvanic series chart The rate at which corrosion takes place is determined by : This means that the stainless steel can attack the nickel in the tungsten carbide causing it to corrode. Passivated 316 stainless steel is positioned nine rows from the bottom. You'll note that active nickel is located twenty one rows from the top of the chart. When this face material is used in dual seal applications it is common to circulate water or antifreeze between the seals (as mentioned in the beginning of this paper, water can be an excellent electrolyte because of the addition of chlorine and fluorine). Nickel base tungsten carbide contains active nickel. In other words the zinc is further away from the bronze than the iron, so the galvanic action takes place between the zinc and the bronze, rather than between the steel and the bronze. Again, looking at the chart, you'll note that zinc is found on line three from the top of the chart. The classic way to solve this problem is to attach sacrificial zinc pieces to the hull and let the bronze go after them. Sea water is a perfect electrolyte, so the bronze fittings would immediately attack the steel hull unless something could be done to either protect the steel ,or give the bronze something else to attack. Note that steel is located seven lines from the ANODIC end, and bronze is listed at twenty seven rows from the same end. The further apart the materials are located on this chart, the more likely that the one on the ANODIC end will corrode if they are both immersed in any fluid considered to be an electrolyte.Ī ship has lots of bronze fittings and a steel hull. I will post a Galvanic Series chart at the end here. Or another example for those with metal (not a dentist so I dont know the exact term) fillings and take a fork which contacts the two metals and the fork being made of a different metal than the fillings, add the saliva, and presto!!! You have a current between them and can sometimes actually feel the tingle.Ī girl in my metals class in college actually asked the prof if that was how the circus clowns light up those lightbulbs when they place them in their mouths. The other metal (the CATHODIC one) will be protected. When the current flows, material will be removed from one of the metals or alloys ( the ANODIC one) and dissolve into the electrolyte.

(This is how the battery in your automobile works). If you put two dissimilar metals, or alloys in a common electrolyte, and connect them with a voltmeter, it will show an electric current flowing between the two. To answer your question briefly.the galvanization is really just a sacrificial coating.(OK I just re-read my post and find it is far from brief and I apologize.anyone not wanting to be bored stiff, if it's not too late for that can just move on) But as I started on this I was thinking of situations with the P-car that you might find relevant such as calcium chloride cindered roadway with water and stainless exhausts etc.anyway here goes. I will try to dig up one of the presentations from ASTM or NASC or find a link to post as the subject can get quite detailed.

Home depot stainless steel bolts