Galvanized and Stainless Steel Bolts Interaction
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Question

Galvanized and Stainless Steel Bolts Interaction

EngineeringEric (Structural)

(OP)

2 Sep 13 09:48

I am working on two projects that I am debating the use of Galvanized steel plates and Stainless Steel Bolts.... Both cases will be considered marine and will be in cyclic conditions of wet/dry.

1) Wet, exposed, bridge. I want to use 1/4" plate connections with stainless through bolts to timber members. I have experienced overly rusted bolts and compromised timber in the past but the plates hold up quite well...

2) similar to first only more complex structure and now we introduce the presence of chlorine (like heavily chlorinated pool water)

... So any issues with material interactions that I am not aware of, or something i can specify to eliminate said interactions? and does the presence of chlorine effect the annodic-cathodic relation ship?

Both of these are large projects and the use of epoxy coatings will destroy the budget and the owner would rather just invest in a more routine maintenance program... but we all know how those maint. programs go.

Thank you for any advice/knowledge

TVP (Materials) · Answer 1 · 2013-09-02 22:52:03

For case 1) you may want to consider hot dip galvanized fasteners, depending on a lot of variables such as maintenance procedures, joint details, etc. Here are some links with more information:

http://www.nickelinstitute.org/~/Media/Files/Techn...
http://www.hdgasa.org.za/Journals/indSearchs/B/Bol...
http://www.galvanizeit.org/images/uploads/publicat...
http://www.standard.no/PageFiles/1194/M-001.pdf

If you decide to use SS, you will need Type 316 (ASTM A193 B8M), also known as A4 according to ISO 3506-1.

For case 2) you will need to look into more highly alloyed grades of SS than 304 or 316. I doubt that galvanized steel plates will be sufficiently corrosion resistant:

http://www.nickelinstitute.org/~/Media/Files/Techn...
http://www.bssa.org.uk/topics.php?article=18
http://www.imoa.info/moly_uses/moly_grade_stainles...

EngineeringEric (Structural) · Answer 2 · 2013-09-03 10:21:21

Thank you TVP, I will look through these soon. And i appreciate you providing the SS grade information.

The second project is for a facility in which my firm has done annual inspections for 15+ years. The structures all utilize painted galvanized plates. The history has shown that the maint. routines of repainting the plates works well and is far cheaper than special alloys. the plates are typically oversize allowing for a large amount of rust before ever compromised. And in reality i would say the most i have seen in area loss to be within less than 5% initial. The bolts however.... they are horrible i have hit one with a hammer and it fell out, the middle of the bolt was being held together with a strand of rust no X-section area.

I am guessing the ongoing maintenance allows the plates to last, but the environment within timber members deteriorates the bolt (hence using SS)

TomDOT (Materials) · Answer 3 · 2013-09-03 16:02:00

I'm never happy with galvanized and stainless in contact - especially in your conditions. The stainless will accelerate the corrosion of the galvanized plates around the contact area.

stanweld (Materials) · Answer 4 · 2013-09-03 17:20:50

You must consider the probability of crevise corrosion if you use austentic stainless steel bolts and you appear to have a number of crevise locations. 316 is just as subject to crevice corrosion as is 304.

EngineeringEric (Structural) · Answer 5 · 2013-09-03 17:30:04

I was thinking of using heavy galv. washers with the intent that they would be sacrificial yet easy to see and replace (relatively). now this does not address the bolt bearing to the plate but i was hoping that would not be too critical. most of the plates are not in a pure shear situation as the frames were designed for positive bearing.

What I was planning, and please throw me under the bus if this is a worse idea than all galv. connections, just A36 HD Galv plate with epoxy paint coating with Stainless Steel, Type B8, Class1, AISI 304 bolts... most are 5/8-3/4" but the first job will have some 1"+ for the heavy timber...?

Rich2001 (Mechanical) · Answer 6 · 2013-09-03 22:15:59

Another concern with this combination is solid metal embrittlement of the stainless steel by the zinc.

Richard

stanweld (Materials) · Answer 7 · 2013-09-04 08:20:00

EngineeringEric,
Have you considered other alloy bolts: e.g., aluminum bronze, K Monel, 17-4 PH, etc. Also were the original steel bolts bare or galvanized? If bare, consider galvanize.

TomDOT (Materials) · Answer 8 · 2013-09-04 09:16:23

If you do spec "galvanized" - be careful how you spec it. Hot-dip galvanizing of a proper thickness is much more corrosion resistant than a thin layer of Zn electroplate (which some do argue is "galvanizing")

We tend to spec ASTM A153 for structural nuts and bolts. We also have an additional requirement if "cold galv" (paint) is used for repair of any galvanized items: a minimum of 94% metallic zinc by weight in the dry film. There are at least 3 common brands which can meet this spec, I think ZRC is the most common - and they have both shiny and dull grey formulations.

ASTM reduced their minimum metallic zinc requirement some years ago for these repair paints, but our in-house testing showed better performance with the 94% minimum.

CoryPad (Materials) · Answer 9 · 2013-09-04 09:23:46

The traditional standard for fasteners was ASTM A153/A153M Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, but it has been replaced with ASTM F2329 Standard Specification for Zinc Coating, Hot-Dip, Requirements for Application to Carbon and Alloy Steel Bolts, Screws, Washers, Nuts, and Special Threaded Fasteners.

TomDOT (Materials) · Answer 10 · 2013-09-04 09:39:21

Interesting, as A153 is still "live" and last updated/renewed in 2009. Any particular advantages to shifting over to F2329?

TVP (Materials) · Answer 11 · 2013-09-04 09:59:34

Here is a good summary of the differences, and why it is recommended that fasteners use the newer F2329 standard:

American Galvanizers Association
Galvanizing Insights Volume 8 Issue 2
http://image.exct.net/lib/ff3716717460/d/1/Dr%20Ga...

CoryPad (Materials) · Answer 12 · 2013-09-04 10:03:29

Note that ASTM F2329 is for fasteners, while ASTM A153 is still active for other hardware applications.

EngineeringEric (Structural) · Answer 13 · 2013-09-04 10:24:56

The old bolts were galv. not sure on the process whether plated or HD. I nearly always require Hot dipped. One of the reasons i am leaning towards the interaction of zinc plate and SS hardware is that the area of zinc for the plate is so large in comparison to the area for the bolts that the cathodic relationship is negligible in a gross area sense... and the smaller bolts will not rust within the timber sections... there mas be pitting and SCC effects but it is my opinion that this should be most visible on the exposed surfaces which are in compression and made of thicker material.

I am still debating on requiring an insulating buffer washer but i think it is a pointless expense as the bolt will always come into contact with the steel plate during install...

Thanks for the resources and helpful info, you have swayed me to look into this further!

TomDOT (Materials) · Answer 14 · 2013-09-04 11:08:09

The anodic/cathodic relationship is important, but I think it is often overblown in situations like yours. Most of the galvanic corrosion will be concentrated in the area within a couple of inches of the bolts.

EdStainless (Materials) · Answer 15 · 2013-09-04 13:21:27

I am concerned about your application #2.
If this is a heavily chlorinated environment with any organics (like in a pool) you will get volatile chloramines.
These will cause SCC of stainless at ambient temperatures.
If you want more info look into the history of swimming pool roof failures.
Never use SS in stressed locations above pools (or any chlorinated water in an enclosed situation).

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Plymouth Tube

EngineeringEric (Structural) · Answer 16 · 2013-09-04 14:51:00

EdStanless, Case two is not a pool house, It is an exterior application which will be under an water attraction. there will be constant spilling of water on it 6 months of the year. I am not sure if it matters being exterior vs interior of a pool... I am guessing exterior will allow for clean water to run over all metals during rain events... not sure if that is good or bad.

Would you recommend just sticking to all HD Galv bolts? I guess I know any application will result in metal failures and deterioration i am trying to determine which one is better in a length of life.

Thanks again everybody!

TomDOT (Materials) · Answer 17 · 2013-09-04 15:23:20

Hm, can your budget handle plates and bolts of silicon bronze for case two?

HD galv is likely the cheaper choice, and may well work "well enough"

EngineeringEric (Structural) · Answer 18 · 2013-09-04 15:45:05

I believe Silicon Bronze runs around 20$ a pound as compared to HD galv steel 1-2$ a pound i think it will break the budget. We have a few hundred 8" angles a large number of tube steel, WT sections, and a few W sections. I am also reusing some sections for an existing mechanical system that we will be disassembling, cleaning, sending off to be galvanized, and re installing so i don't want to add a third metal to my pot. But i will keep that in mind for future projects (at least store it in the Rolodex for future examination).

I was hoping SS bolts would be a magic bullet since the existing galv. bolts are in horrible shape.

TomDOT (Materials) · Answer 19 · 2013-09-04 16:13:35

If you have a local galvanizer, periodic regalvanizing of larger items and replacement of HDG bolts sounds like a reasonable approach.

EdStainless (Materials) · Answer 20 · 2013-09-04 18:19:25

I would stick with HDG for all of it.
Rain is good, it washes away crap that settles, especially near salt water.
You might want to look and make sure that you don't have places that can trap water though.
How the joints are made up can matter, often it is scuffing from tools or nuts (or bolt heads) turning against plates that scrape the Zn and create places that start to rust first.

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Plymouth Tube

TVP (Materials) · Answer 21 · 2013-09-05 07:42:49

I would be inclined to go with HDG too, based on your budget concerns. I have concerns about Type 304 stainless in this situation, and even 316 is marginal for crevice corrosion. The links I provided on swimming pools shows austenitic alloys that are considerably more expensive than even 316.

EdStainless (Materials) · Answer 22 · 2013-09-05 09:15:58

The trend for infrastructure work is to use duplex stainless grades.
They are about the same price as 316, but much stronger.
While they have similar crevice corrosion resistance to 316 they do have significantly better SCC resistance.
Grades like 2101, 2003, 2102 are becoming more popular for rebar, but they haven't worked into hardware like you are using yet.
The traditional duplex, 2205 is more expensive but it has higher corrosion resistance.

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Plymouth Tube

TVP (Materials) · Answer 23 · 2013-09-05 15:28:30

FWIW, Bumax Stainless in Sweden manufactures fasteners using duplex grade 2101. They aren't off-the-shelf items, but it is possible to obtain them:

http://www.fera.org.uk/pdf/Fera%20seminar%202%20Fe...

http://www.bumax.se/wp-content/uploads/2013/06/BUM...

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Galvanized and Stainless Steel Bolts Interaction
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