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Sea water couple, SS316 & SS304 galvanic corrosion

Q. I would like to know whether SS316 and SS304 will form a galvanic couple in sea water slow velocity service. If the corrosion protection is by means of cathodic protection, is it correct to assume that the less noble material will be corroded to the extent of irreparable condition within very short interval of time i.e few months time in case CP fails to operate. Appreciate your answer for any further measures to be taken for other equipments in case CP had failed and takes some time to bring in line.

MAMIDI GOIVNDA RAJU
Mechanical Engineer - Bintulu, Sarawak, Malaysia
2004

A. 304 Stainless Steel should not be considered suitable for chloride environments and begins pitting immediately. In this condition, SS 304 becomes anodic to SS316. As a rule of thumb, if the potential difference is greater than 0.1 volt, then the galvanic corrosion becomes significant. Typically the corrosion potential difference between corroding SS304 and non-corroding SS316 is approximately 0.3V. Therefore, SS316 will accelerate the corrosion of SS304 by galvanic corrosion cell.

Miki Funahashi
- USA
April 20, 2011

A. It depends if for example a 316 anchor is used to hold a 304 plate. in this case electrons flow from the 304 to 316 which is nobler. But the cathodic surface (bolt) is small compared to the plate hence there is no corrosion. In the reverse, i.e., a 304 anchor is used to hold a 316 plate, the cathode area is larger and corrosion would take place. However, it depends on water or electrolyte presence.

William Hooi
- kl Malaysia
July 1, 2014

Type 304 or 316 for Wine-making?

Q. I am a Portuguese oenologist and I'm buying stainless steel casks for my cellar. The person who is selling the stainless steel advises me to buy all in 304 because some countries like EUA don't allow manufacturing the casks in 316, and they say that we could be in trouble in the future when we export to EUA. I would like to know what are the advantages and the disadvantages of using 304 or 316.

Diogo Pereira
winemaking - Portugal
2005

A. Only for mechanical resistance can 304 be considered slightly better than 316. For corrosion purposes 316 is better.

Guillermo Marrufo
Monterrey, NL, Mexico
2005

A. Hi Diogo. I do not believe that that particular statement is true.

But your question really involves winemaking standards rather than general corrosion questions, and I have read on multiple sites that 304 is adequate for red wine, but that for white wine type 316 is strongly recommended due to more acidity and more sulphates.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Material selection: 304 vs 316 stainless

December 30, 2010

Is there any reference material which can be referred to for deciding which Material Grade to use for a particular SERVICE?

For example: SS 304 Vs SS 316.
I was told by some industry guy, that SS 304 is used for ATMOSPHERIC CORROSION SERVICE
Whereas for Food Grade FLUIDS we must use SS 316!

But my feeling is that " Selection must be based on certain sound rules ".

Listing down the certain CRITERIA :
1. STRENGTH
2. SERVICE
3. ENVIRONMENT
3. COST
4. AVAILABILITY

Of these , Strength decision can be made from Design Codes, COST and AVAILABILITY are commercial and available from market.
This leaves SERVICE & ENVIRONMENT which come from the chemistry. But the DESIGN CODES are silent on CHEMISTRY of metal selection !

So if any person is having any idea about reference for MATERIAL Selection (relevant to POWER Sector), please do share.

regards,
DC

Deepankar Chatta
Designer - India

"Chemical Resistance Guide For Metals And Alloys"
by Kenneth pruett

on AbeBooks or
Amazon

(affil link)

A. For chemical resistance, I recommend "Chemical Resistance Guide for Metals & Alloys" by Compass Publications.

Jon Barrows, MSF, EHSSC
Kansas City
December 30, 2010

A. Hi Deepankar. 316 is more corrosion resistant in all environments than 304, and is sometimes required even for atmospheric service (railings very near the sea for example), but 304 is usable for many liquid containment applications (see red wine casks above) too. Environment is paramount.

But I would also add that for ultra-purity in the food and pharmaceutical industries, electropolished (free of burrs, nooks, and crannies) type 316 is often specified.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

adv.

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