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When Are Stainlesses "Active" And When Are They "Passive"?Helpful Member!(5) 

tc7 (Mechanical) (OP)
7 Jul 08 14:21
Just about every Galvanic corrosion chart that is published lists 430, 302, 303, 321, 316, 317, 347, 410 and 416 stainlesses on both sides of the chart, but lists them as either "passive" or "active"....how do we know if it is passive or active?

And where would a PH stainless (in particular 17-4) fall on the chart?

I am trying to determine of a 17-4Ph sensor threaded into a 4340 carbon steel structure, employed in an oceanside or shipboard environment will be an acceptable couple?

Thankyou.  
Helpful Member!  gr2vessels (Mechanical)
7 Jul 08 21:20
tc7,
Any grade of stainless steel compared with carbon steel is far away on the galvanic corrosion scale, especially in a strong electrolitic environment like salty water or salty moisture.
Depending on the mass of the carbon steel structure, the galvanic corrosion will occur and as consequence the stainless steel material will be corroded away. Depending on other factors like temperature, concentration, presence of oxygen in the electrolyte, etc, the corrosion could be severe and rapid. All this can be avoided easily by trying to isolate electrically the parts (plastic bushing between the parts), putting heavy paint on the stainless steel component to isolate it from the electrolyte, or simply replace the 17-4 sensor with a carbon steel one.
As far as the "active" and "passive" stainless steel topic, there is google.
cheers,
gr2vessels
Helpful Member!  metengr (Materials)
7 Jul 08 22:53
For stainless steels that are exposed to aerated, flowing water conditions, the corrosion potential would be passive (noble). For conditions where you have crevices or oxygen depletion in stagnant water, the less noble corrosion potential should be used (active).

In general, the martensitic precipitation hardening stainless steels as a class have comparable corrosion resistance to the more common grades of austenitic stainless steels (more noble as stated above unless you have crevices) in most media. This is an excerpt from Handbook of Stainless Steels.
Helpful Member!  strider6 (Materials)
8 Jul 08 4:25
Stainless Steel are "usually" in the passive state. They may become "active" when, as someone else said, they are in an enviroments with  strong reducing conditions, without oxygen for example, or very oxyding condition,  or when they are in environments with high content of chlorides..
The passive layer if damgaged repair itself but if there is no oxygen, exmaple in stagnant water, this passive layer can't reapir and the stainless steel become active and corrode.
In your case you'll ahve the potential for galvanic corrosion, expecially 'cause you're in a marine environments.
In my opinion you should look carefully at your material selection...in a marine environments i'll select a 316 that is more resistant to chloride corrosion than 17-4 ph.

When is stainless steel passive or active - formation of the passive layer
http://www.bssa.org.uk/topics.php?article=16

Galvanic Corrosion
http://www.ocean.udel.edu/mas/masnotes/corrosion.pdf



S

 

Metal Corrosion
 

Helpful Member!  EdStainless (Materials)
8 Jul 08 9:38
IF he is using 17-4 PH then he must need some strength.  316 is soft and has poor crevice corrosion resistance in seawater.
You should look at the duplex 2205 as a starting point.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

tc7 (Mechanical) (OP)
8 Jul 08 9:43
So then I would imagine that the passive oxide layer on a SS threaded component would be quickly swiped away as it is threaded and torqued into its mating part; therfore leaving the SS in an active state and quite very far apart on a Galvanic chart from the 4340 carbon steel structure, as mentioned by gr2vessel.

Mention has been made that the SS material would corrode away, but observation on similar assemblies is that the carbon steel is more corroded and loss of material is greater on the mating carbon steel threads.  This seems opposed to the theory, how would this be explained?  
tc7 (Mechanical) (OP)
8 Jul 08 9:57
EdStainless-
You are correct that strength is required as this is for some type of load cell.  I do not yet have an idea of the total load on the threads of the device, but am certain that I may not change the 17-4 material.  Thanks.
strider6 (Materials)
8 Jul 08 11:50
if 316 has poor resistance i can't imagine how much is the resistance of a 17-4!! Here we're talking of atmospheric exposure or immersion in seawater??
 

Metal Corrosion
 

EdStainless (Materials)
8 Jul 08 12:31
Yes, the CS will be the part that corrodes.
When the SS is actively corroding its potential will be very similar to CS.  When the SS is passive the potential is quite different.
Since 17-4 has no Mo it will behave like other 17% Cr grades, 304 and 439.
You need to look into sealing the threads.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

tc7 (Mechanical) (OP)
8 Jul 08 14:57
My hope is to repair the corroded threaded holes by drilling and grinding out to remove all corrosion, then clad welding a layer(or layers) of ER630, ER2209, or even ER420 to restore all lost material on the 4340 carbon steel structure then drilling and tapping for the same old 17-4 sensor.

What are your thoughts on this idea? would this beat the galvanic corrosion problem?
metengr (Materials)
8 Jul 08 22:17
tc7
I would not recommend you re-weld using a dissimilar metal weld. Plug weld the hole using a low alloy filler metal, drill and re-tap the hole for the 17-4 PH sensor. I would use a sealant on the threads.
metengr (Materials)
8 Jul 08 22:20

Quote:

I would not recommend you re-weld using a dissimilar metal weld.

Why? Because you still have a region of dissimilar metals coupled together now called a weld fusion line that is susceptible to knifle line corrosion damage.
gr2vessels (Mechanical)
8 Jul 08 22:47
Partial cladding of carbon steel with 17-4 would only shift the galvanic corrosion from the threaded connection to the boundary of welded overlay exposed to the wet marine environment. However, it's likely that the corrosion of the thread on the sensor and the cladded structure will be minimized (if any, except for some galling potential). It is a quick fix and very practical, but not long term solution.
cheers,
gr2vessels
strider6 (Materials)
9 Jul 08 3:42
This is my point of view:

1) Stainless Steel coupled with carbon steel in a marine environments is a galvanic cell and you've to expect corrosion
2) 17-4 ph is not a stainless steel to be used on a marine environments or you0've to paint it and make future maintenance.
3) If we are talking of external corrosion then just paint both of the materials make maintenance and you'll get rid of corrosion
4) if we are talking of corrosion due to the internal fluid then you've to change the materials or find other solutions...

S.

Metal Corrosion
 

Helpful Member!  0707 (Petroleum)
9 Jul 08 10:49


"Stainless steels each occupy two locations in the galvanic series of metals. Stainless can be both passive and active.
The same piece of stainless can be both in different parts. Stainless is usually passive when it has enough oxygen
available to create a tough oxide on the surface. Stainless usually becomes active when there is insufficient oxygen, as
when embedded in sea water soaked wood."

luismarques
tc7 (Mechanical) (OP)
9 Jul 08 12:09
Thanks folks, very good information from many points of view. Stars for all!!!

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