When Are Stainlesses "Active" And When Are They "Passive"?
When Are Stainlesses "Active" And When Are They "Passive"?
(OP)
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.
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.





RE: When Are Stainlesses "Active" And When Are They "Passive"?
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
RE: When Are Stainlesses "Active" And When Are They "Passive"?
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.
RE: When Are Stainlesses "Active" And When Are They "Passive"?
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
ht
S
Metal Corrosion
RE: When Are Stainlesses "Active" And When Are They "Passive"?
You should look at the duplex 2205 as a starting point.
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Plymouth Tube
RE: When Are Stainlesses "Active" And When Are They "Passive"?
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?
RE: When Are Stainlesses "Active" And When Are They "Passive"?
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.
RE: When Are Stainlesses "Active" And When Are They "Passive"?
Metal Corrosion
RE: When Are Stainlesses "Active" And When Are They "Passive"?
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.
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Plymouth Tube
RE: When Are Stainlesses "Active" And When Are They "Passive"?
What are your thoughts on this idea? would this beat the galvanic corrosion problem?
RE: When Are Stainlesses "Active" And When Are They "Passive"?
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.
RE: When Are Stainlesses "Active" And When Are They "Passive"?
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.
RE: When Are Stainlesses "Active" And When Are They "Passive"?
cheers,
gr2vessels
RE: When Are Stainlesses "Active" And When Are They "Passive"?
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
RE: When Are Stainlesses "Active" And When Are They "Passive"?
"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
RE: When Are Stainlesses "Active" And When Are They "Passive"?