Corrosion of SpringsThe Role of Corrosion in Premature Failures

And the Means to Prevent Those Failures

By

Dr. Kent Johnson, P.E., FASM,

Robert O’Shea, Jr., P.E.

Luke Zubek, P.E.

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Two Most Common Causes of Spring Failures

1. Fatigue (Progressive failure - Fatigue defects grow over time).

2. Embrittlement (Abrupt or Delayed Failure - Hydrogen Damage)

Progressive Failure Mechanisms

1. Loading in the elastic range is required for failure.

2. Cracks grow progressively larger over time, up until a critical size is reached and failure of the spring occurs.

3. Both Fatigue and several types of Corrosion are progressive failure modes. When Fatigue is combined with a progressive mode of Corrosion, very early premature failures of springs can occur.

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Corrosion Definitions

1. The progressive deterioration, embrittlement or destruction of a metal by chemical action.

2. The progressive embrittlement or destructive attack on a metal through interaction with its environment.

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Reason for Corrosion

1.Metals are obtained by applying massive amounts of energy to mined ores, increasing that extracted metal’s potential energy.

2.Corrosion is the mechanism that reclaims that energy over the life of the metal, and attempts to return the metal to its original state as an oxide.

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Two Major Classifications of Corrosion

1. The spring is exposed to a liquid environment (i.e., wet or aqueous corrosion).

2. The spring is exposed to a gaseous environment (i.e., dry or atmospheric).

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Corrosive Mechanisms

1. Uniform Corrosion.

2. Pitting (cracking common at bottom of pits). Progressive

3. Selective Leaching. Progressive

4. Intergranular Corrosion.

5. Crevice Corrosion.

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Corrosive Mechanisms (Continued)

6. Galvanic Corrosion.

7. Erosion-Corrosion.

8. Stress Corrosion Cracking (SCC). Progressive

9. Hydrogen Damage Failures. Progressive

10.Liquid Metal Embrittlement. Progressive

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High Cycle Fatigue Life of a Spring

Overall Fatigue Life is a Combination of:

1.Crack initiation number of cycles (about 50% to 90% of the total fatigue life) 90% is the most conservative.

2.Crack propagation number of cycles (about 10% to 50% of the total fatigue life) 10% is the most conservative.

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Fatigue Life of a Spring Example of Fatigue Life Estimation:

1” diameter bar with 0.75” fatigue and 0.25” overload. SEM Examination revealed that the striation spacing averages 1 micron or about 18750 cycles for crack propagation.

10% * Crack initiation = 18750 cycles

Crack initiation = 187500 cycles

Total Cycle Life Estimation = 187500+18750= 206250 cycles

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Elimination of Fatigue Initiation

• Corrosion can reduce the overall fatigue life of a spring by facilitating crack initiation, which accounts for ~90% of the fatigue life.

• Processing that reduces the residual surface tensile stresses, like shot peening or stress relief, effectively increases the fatigue life through this mechanism.

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Effect of Corrosion on Fatigue (i.e., Corrosion Fatigue)

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• Normal fatigue limit

• No limit in the presence of

corrosive agents A and B

Corrosion Fatigue and Spring Service Life

1. All forms of corrosion fatigue lower spring service life. Some a little, some a lot.

2. For a large number of cycles to failure, corrosion fatigue can lower a spring’s fatigue strength (endurance limit) up to 75 percent.

3. For a given stress level, corrosion fatigue can lower the number of cycles to failure by over 100 times (25 yrs to 3 months or less).

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Tools for Diagnosing Corrosion

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Scanning electron microscope (SEM) with Energy Dispersive Spectroscopy (EDS)

Tools for Diagnosing Corrosion

Inverted metallurgical microscope

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Effect of Fatigue Frequency of Load Application

1. In non-corrosive environments, cyclic loading frequency generally has little effect on fatigue behavior of a spring.

2. On the other hand, fatigue behavior is strongly dependent on frequency in corrosive environments. The corrosion fatigue strength (i.e., fatigue endurance limit) decreases with decreasing frequency and the fatigue crack propagation rate becomes faster at low frequencies.

3. Also, the more corrosive the environment, the lower the corrosion fatigue strength (i.e., fatigue endurance limit) decreases with decreasing frequency and the fatigue crack propagation rate becomes faster at low frequencies.

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Control of Corrosion Fatigue

1. No metallic spring is immune from some reduction of its resistance to cyclic stressing when placed in either a wet or a dry corrosive environment.

2. Control of Corrosion Fatigue can only be accomplished by either:

a. Lowering the cyclic stress intensity

b. By various corrosion control measures

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Corrosion Control Measures

1. Design & Material Selection (i.e., Metallurgy)

2. Fabrication & Processing of Spring Material To Improve Corrosion Resistance

3. Corrosion Testing, Salt Spray Testing

4. Protective Coatings and Corrosion Inhibitors

Note: Listed in order of importance.•

Design & Material Selection (i.e., Metallurgy)

• Environment

• Stress

• Compatibility

• Movement

• Temperature

• Control

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Dip Spin Coatings for Corrosion Protection

• Dip-Spin Coatings. The greatest change in fastener finishing for automotive applications is the increasing acceptance of so-called dip-spin finishes. This is not new technology, but dip-spin coatings have been improving and increasing their share of the market for fastener finishing.

• These coatings provide better corrosion protection than zinc electroplating without the possibility of hydrogen embrittlement.

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Where the industry is Going?

Geomet® and Dacromet® coatings: chrome free coatings that are not susceptible to hydrogen embrittlement.

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Fabrication & Processing of Spring Material

To Improve Corrosion Resistance

1. Painting (after coiling)

2. Zinc-Phosphate Coating (after coiling)

3. E-coating (after coiling)

4. Dip Spin Coatings (after coiling)

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Protective Coatings and Corrosion Inhibitors

• - Inhibitors

1.CPCs - Phosphates, Chromates, Nitrates, and Molybdates

2.Oxidizers

3.Amines & Hydrazines (organic)• Coatings

1.Organic

2.Zinc or cadmium

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STOP THE CORROSION – STOP THE PREMATURE FAILURES

1. Corrosion fatigue at low frequency has a greater effect in decreasing spring life.

2. Initiation and propagation rate of corrosion-fatigue cracks in service are increased by corrosive environments, mainly, bulk aqueous solutions and environments produced by continuous and periodic vapor condensation on the affected surfaces.

3. The fatigue strength, or fatigue life at a given value of maximum stress of any spring, generally decreases in the presence of an aggressive environment.

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Zinc Plated Spring Used in Seawater (i.e. Wrong Material Selection)

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Zinc Plated Spring

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The CrSi spring was corroded and fractured in multiple locations.

The root cause of the failure is improper material selection (not corrosion); a

better choice would be 302 SS.

Propane Truck Valve Spring

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Propane Truck Valve Spring

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Propane Truck Valve Spring

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Propane Truck Valve Spring

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Propane Truck Valve Spring

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Propane Truck Valve Spring

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302 Stainless Steel Spring

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302 Stainless Steel Spring

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302 Stainless Steel Spring

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302 Stainless Steel Spring

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This stainless steel spring was corroded and fractured in multiple locations.

The root cause of the failure is improper material selection (not corrosion); a better choice would be Inconel

X750.

Linear seams were

found to be detrimental

Solution:

An Inconel compression spring was utilized.

Solution: Electro-polish

Thank You – Any Questions

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