chloride scc

7/30/2019 Chloride SCC

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John DimmickDirector of Technology

www.Cleanvehicle.org

Factors Aggravating Chloride Stress Corrosion Cracking

In Austenitic Stainless Steels

October 31, 2012

Purpose:

This is intended to be a resource for choosing and qualifying materials and components to beused in vehicle gaseous fuel system components. It is a response to reports of in-service failures

of natural gas vehicle components made from austenitic stainless steels, Type 303 and 304, whenexposed to deicing salt environments.

The primary source for this information is NBS Monograph 156, Stress Corrosion Cracking

(SCC) Control Measures

Chloride Stress Corrosion Cracking (SCC):

Chloride SCC is suspected to be the failure cause of both 304 stainless steel wire reinforced

hoses and 303 stainless quick disconnect nipples while exposed to road salt on natural gas

vehicles. The primary causative factors for this SCC are:

1. Chloride ions in aqueous solutions or in solutions containing water or perhaps other

electrolyte

2. Sustained tensile stress

3. A susceptible steel

4. Oxygen or other oxidizer

5. Elevated temperature

6. Favorable electrochemical potential

7. Opportunity to develop local acidity

8. Opportunity for local breakdown of passivity.

Chloride SCC Test:

The standard test for resistance to chloride SCC uses a hot magnesium chloride solution and thisis useful to rank independent variables but there is no known correlation of this test to

automotive service environments. It is likely that this test for industrial chemical industries is

much more severe than an automotive exposure. It must be assumed that chlorides are present in

any external automotive installation, especially underbody.



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Factors:

The threshold stress for SSC of susceptible stainless steels is so low, probably under 10ksi for

steel with a tensile strength of 90 ksi that “…reliance for control of chloride stress corrosion

cracking is largely placed in the control of one or more of the other factors.”

The maximum susceptibility in the MgCl test is in the range of 8% -12% Ni. This is exactly the

range for 304 stainless steel while 316 in the US has a minimum of 10% Ni. Some other

countries require 12.5% Ni for their equivalent “316”.

Cold work increases susceptibility and braid wire is heavily cold drawn to increase strength.

There are a number of alloy tweaks and micro-alloying elements that can be used to enhanceresistance to SCC but only the base specifications are considered here.

Phosphorus and Nitrogen are detrimental as is Molybdenum.

Carbide precipitate sensitization such as occurs in a heat affected zone of a weld is verydetrimental but if no such heating occurs, high-side carbon content is thought to be beneficial.

In general, the austenitic alloys can be grouped into four “…non-quantitative categories of

relative susceptibility as follows” from high susceptibility to low.

1. High susceptibility: Sulfur bearing 301, 301 and sensitized 304

2. Intermediate susceptibility: Non-sensitized 304 and 304L

3. Lower Susceptibility: 316, 316L, 347, 317L

4. Lowest susceptibility: 310, 314, USS 18-18-2

The time to failure in hot MgCl is very roughly about four times greater for 316 than for 304 atlikely operating stress levels.

SCC of these steels seldom occurs at temperatures below about 140F but an instance of RTfailure in bleach is reported. Temperatures under vehicles can exceed 140F and since the hose

has little thermal mass, it could experience higher temperatures.

Local acidification and compromised passivity are connected since chlorides in the presence of

acid can penetrate the passive film. Local acidification is promoted by a shelter such as a creviceor wet insulation or “poultice effects” such as under sand or a paint blister. Acid rain could also

be a factor and acceleration by the presence of sulfur compounds from industrial or vehicleexhausts have also been implicated in SCC failures.

The following factors present in the failed hose and its probable environment are thought toincrease the likelihood of chloride SCC.



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The Ni content of 304 falls within the susceptible range.

Cold work increases susceptibility and the wire is probably not annealed after drawing so that it

has a useful yield strength.

Wet-dry and hot-cold cycles under the vehicle can concentrate salt deposits.

Wicking of salt solutions is a common contributing factor and this may resemble the semi-

covered braid.

Contamination with rust from other steel members can accelerate SCC and would be common

under a vehicle exposed to deicing salts. Photos of sister vehicles show chassis rust and this is probably unavoidable.

SCC is promoted at temperatures above RT. The under-vehicle ambient temperatures can

approach the NGV2 maximum of 180F on hot days.

Penetrations of the passive film can result in SCC. It seems likely that wire-on-wire wear in the

braid could easily remove the passive film.

An acid pH increases SCC susceptibility by penetrating the passive film. Many locales are still

in an acid rain band and corrosion by acidified road salt is a very aggressive environment.

Present Required Component Tests:

NGV4.2 has an external fluids exposure test with similar chemicals to the NGV2 Environmental

Test but unlike NGV2, the hose is not pressurized for an extended period of time while exposed to the chemicals. This means that susceptibility to failure mechanisms such as SCC is not tested.

Conclusions:

1. Tentatively the failures of 304 wire braid under vans in Connecticut, Michigan and

Oklahoma can be considered probable chloride SCC pending laboratory analysis.

2. The NGV4.2 environmental test is not intended to fail stainless steels that are susceptible

to chloride SCC.

3. A developed and validated accelerated test for austenitic stainless steel under-body

components to verify resistance to chloride SCC has not been identified.

4. Considering the difficulty of developing and validating a component test and the small

quantities of NGV and HGV components, a design specification rather than a new test is

considered more feasible.



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5. The existing 304 wire hoses have sufficient resistance to SCC that only seven failures out

of thousands of hoses are known and these took a few years to manifest themselves. This

suggests that incremental improvements to the factors contributing to SCC could be

effective in preventing future failures.

6. The SCC failure of the 303 valve bodies was probably the result of very high stresses

resulting from tightening the NPT connections.

Recommendations:

1. Secure a competent laboratory analysis of the failed hose braid to determine if the failure

involved SCC and also the chemistry and properties of the failed wire.

2. Add requirements to the specifications for under-body hoses in order to reduce the

susceptibility to SCC.

a. Prohibit the use of an external jacket or film label that may trap, sustain and

concentrate harmful environments.

b. Increase the minimum nickel content above the nominal range of 304. The 316

range may be adequate if it is determined that the failed wire was at the low end

of the 304 range.

c. Require heat shielding of hoses from the exhaust system.

d.

Require that all exterior surfaces be in a passive condition.

e. Prohibit the use of NPT tapered pipe threads in austenitic stainless steel

components.

3. While these changes should reduce the likelihood of future failures due to SCC we could

perform a rough evaluation of the results by subjecting examples of the new and old

hoses to an SCC test based on an automotive acidified, wet/dry salt corrosion cycle test.

This test could be performed at somewhat higher temperatures and stresses that actual

hose service in order to force failures. All that we might learn is that the new

requirements produce a hose with increased durability and resistance to SCC.

4. The alternative of a performance test for resistance to SCC could be considered as an

alternative if a test were to be developed and validated to the satisfaction of the approval

agency.

chloride scc

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