Letter 38035

Galvanic corrosion of alloy steel fasteners in stainless  

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We are experiencing alloy steel bolts (grade 10.9) corroding and breaking in large stainless steel assemblies being used in the food industry. The environment is hot (200 F) and humid with caustic solution occasionally being used to wash the equipment down. The bolts are Geomet Plus coated as this is FDA approved (no hexavalent chrome). The bolt heads are breaking off due, I think, to stress corrosion compounding the galvanic corrosion. Where the moisture is staying in contact with the bolt, the corrosion is worst. I was thinking about assembling the bolts using RTV under their heads to prevent moisture ingress. Zinc Chromate primer is an FDA no no! Would greatly appreciate your thoughts please. Thanks.

Martin Hughes
design consultant - Pittsfield, Massachusetts

A grade 10.9 bolt is extremely hard. I am unsure what the exact material and finish is that you are using on these bolts. If it is a carbon steel bolt with electro-deposited finish you are experiencing a delayed brittle failure due to hydrogen embrittlement. I would investigate the plating specification and proceed from that point. Reference ASTM B-633 [link is to spec at TechStreet] as a guide for preventing this failure. It is unlikely that the bolts are failing due to corrosion. The location you are describing is where delayed brittle failures usually occur.

Do you really need a bolt that is that hard?


Jason

Jason Siewert
- York, PA

The bolt material is required for the tensile properties to provide an adequate safety margin. A2 or A4 stainless bolts are not strong enough. We have been assured that the Geomet coating does not induce hydrogen embrittlement. However, it doesn't appear to help prevent the galvanic corrosion as the zinc is far apart on the galvanic table. Corrosion is prevalent all over the bolt, but mainly in the area where it is kept wet. We were wondering if Armoloy or Xylan coatings would be more appropriate for the mating stainless parts to reduce galvanic corrosion. Armoloy, being thin dense chrome, is closer to stainless on the galvanic table and Xylan is non-metallic.

Martin Hughes
- Pittsfield, Massachusetts

Jason is probably correct, but there is also a possibility of thread seizure.
What is the length of service?
What is the bolt (being used as capscrew) size and the thread engagement length?
What are the torques at installation, failure and for removal of non-failures?

Ken Vlach - Goleta, California

Bulten Stainless of Sweden makes a corrosion-resistant fastener in a high strength, high Mo version of 316L. Available from Unbrako.

"Min. breaking point 1000 N/mm2 and min. elongation limit 900 N/mm2. Bumax 109 products are made from acid-proof steel, SS 14.2343/1.4436, with a low carbon content of max. 0.03% (AISI 316L high Mo) and, as regards breaking point and elongation limit, they are equivalent to Class 10.9 steel screws."

Threads are "anti-friction conditioned" which should minimize potential galling.

Ken Vlach
- Goleta, California

Many thanks for the link to the Bulten stainless screws. That will be very useful as it will probably solve my problem if they have stock!

Martin Hughes
- Pittsfield, MA,USA

While the Geomet coating may not induce hydrogen embrittlement, remember that coating processes involve pretreatment, and if cathodic cleaning or acid activation were involved (as opposed to strictly mechanical blasting for pretreatment) these introduce the possibility of hydrogen embrittlement.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Several years ago I learned of some failures to stainless steel screws due to unexpected causes. These were building facade failures.

Specifically, micro examination of the failed screws revealed micro fractures induced in the screws by the screw manufacturing process. I was told the fractures permitted fluids to reach interior parts of the steel that were not fully stainless.

This suggests two vehicles for failure: 1. the micro fractures weakened the screw, or 2. the microfractures exposed concealed parts of the metal screws that were then subject to rusting or corrosion.

Drake Jacobs
- Cambridge, MA

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