1. Corrosion of Springs
The 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.
2. •
Two Most Common Causes of Spring
Failures
1. Fatigue (Progressive failure -
Fatigue defects grow over time).
2. Embrittlement (Abrupt or Delayed
Failure - Hydrogen Damage)
3. 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.
•
4. 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.
•
5. 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.
•
6. 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).
•
7. Corrosive Mechanisms
1. Uniform Corrosion.
2. Pitting (cracking common at bottom
of pits). Progressive
3. Selective Leaching. Progressive
4. Intergranular Corrosion.
5. Crevice Corrosion.
•
9. 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.
•
10. 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
•
11. 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.
•
12. Effect of Corrosion on Fatigue (i.e., Corrosion
Fatigue)
•
• Normal fatigue limit
• No limit in the presence of
corrosive agents A and B
13. 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).
•
16. 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.
•
17. 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
•
18. 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.•
19. Design & Material Selection (i.e.,
Metallurgy)
• Environment
• Stress
• Compatibility
• Movement
• Temperature
• Control
•
20. 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.
•
21. Where the industry is
Going?
Geomet® and Dacromet® coatings: chrome free
coatings that are not susceptible to hydrogen
embrittlement.
•
22. 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)
•
23. 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
•
24. 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.
•
26. Zinc Plated Spring
•
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.
36. 302 Stainless Steel Spring
•
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.
37. Linear seams were
found to be detrimental
Solution:
An Inconel compression spring was utilized.
Fatigue is addressed in this presentation. Relaxation is not addressed in this presentation. Relaxation is only a progressive form of failure at elevated temperatures where creep is a factor.
Progressive failure mechanisms are the most insidious of all failure mechanisms – the failure occurs at the design service loading, usually in a sudden and catastrophic manner. With a progressive failure mechanism – defects grow in size over time, and when a critical size is finally reached, a sudden failure occurs. Both fatigue and several of the corrosion mechanisms are progressive forms of failure. Whenever two types of progressive failure mechanisms work together (Fatigue and progressive Corrosion), very early and sudden premature failures of springs can occur.
The reason why corrosion exists is that metallic springs are not in the lowest energy state. In order to get metals, we take ores out of the ground and apply tremendous amounts of energy (thermal, electrical, chemical, etc.) to those ores to turn them into metals. This increases the stored energy within the metal by a very large amount. Exposing the metal to it natural environment initiates the corrosion process whereby this energy is reclaimed and the metal is turned back into compounds found in its ores (i.e., oxides, sulfates, chlorides, nitrides, etc.).
Aqueous corrosion – the spring is mainly in contact with the corroding liquid. (Bob – find some good examples)
Gaseous or atmospheric corrosion – the spring is mainly in contact with the vapor of the corroding species. (Automotive leaf and coil springs, pressure relief valve springs, etc.
Note: Spring corrosion mechanisms listed in light blue cause crack-like defects to occur in a short period of time and thereby eliminate the initiation stage of fatigue.
Note: The red progressive classification identifies those spring corrosion mechanisms causing progressive growth of crack-like defects that continue over time. This has the potential to cause very premature progressive types of spring failures (i.e., failures occurring in months rather than in years).
If you want Bob, you can briefly describe these different corrosion mechanisms.
Uniform Corrosion:
Pitting Corrosion:
Selective Leaching:
Intergranular Corrosion:
Crevice Corrosion:
Note: Spring corrosion mechanisms listed in light blue cause crack-like defects to occur in a short period of time and thereby eliminate the initiation stage of fatigue.
Note: The red progressive classification identifies those spring corrosion mechanisms causing progressive growth of crack-like defects that continue over time. This has the potential to cause very premature progressive types of spring failures (i.e., failures occurring in months rather than in years).
If you want Bob, you can briefly describe these different corrosion mechanisms.
Galvanic Corrosion:
Erosion-Corrosion:
Stress Corrosion Cracking:
Hydrogen Damage Failures:
Liquid Metal Embrittlement:
Note that fatigue crack initiation can take almost the entire fatigue life (i.e., 75 to 90 percent of the time to failure). Therefore anything the short circuits this crack initiation period will seriously reduce the overall fatigue life of a spring.
Note that fatigue crack initiation can take almost the entire fatigue life (i.e., 75 to 90 percent of the time to failure). Therefore anything the short circuits this crack initiation period will seriously reduce the overall fatigue life of a spring.
NOTE: the elimination of the crack initiation number of cycles, by generating crack-like defects early in the life of a spring, will immediately reduce the overall high cycle fatigue life of the spring (i.e., failures occurring at the design service loading) to only 10% (25 yrs to 2.5 yrs) to 50% (25 yrs to 12.5 years) of what it should have been.
Corrosion modes # 2, 3, 4, 5, 8, 9, & 10 act to eliminate the fatigue initiation cycle.
Whenever there are no corrosion effects present on a spring that is being loaded in fatigue, a well defined fatigue strength limit (i.e., the endurance limit) defines the alternating stress intensity below which no fatigue failure will occur.
In many cases the amount that corrosion fatigue lowers spring service life is not significant, while in other cases it can have a sudden and catastrophic effect.
Corrosion fatigue can lower the endurance limit by up to 75%. This is the right side of curve B in the previous slides.
Corrosion fatigue can lower the number of cycles (time) to failure by over 100 times (25 yrs to 3 months). This is the left side of curve B in the previous slides.
For EDS, Excellent non-destructive tool
The frequency of the fatigue loading normally does not effect the overall fatigue life of a spring that is not undergoing corrosive effects. Corrosion either eliminates the fatigue initiation cycle, develops progressive crack-like defects that grow larger with time, or yields some combination of these two factors. When corrosion is involved, then lower frequencies are more detrimental to fatigue life than higher frequencies of load application, since corrosion is a rate controlled process (i.e., it takes some time to happen).
The more aggressive the corrosive environment, the greater this frequency effect. Corrosion-fatigue strength (endurance limit at a very large number of cycles) generally decreases with decreasing cyclic frequency. This effect is most pronounced at frequencies of less than 10 Hz.
Low frequencies, especially at low strain amplitudes or when there is substantial elapsed time between changes in stress levels, allow time for corrosive interaction between the spring and its environment. On the other hand, high frequencies do not allow for this type of interaction, particularly when high strain amplitude is involved.
There are only two means to control, or limit the effect of corrosion fatigue.
Lower the cyclic stress intensity level.
Eliminate or reduce the effect of the corrosion – Corrosion Control.
1. Design/Metallurgy
2. Fabrication & Processing
Note: Both items 1 and 2 can affect the lowering of the cyclic stress intensity as well as provide various corrosion control measures
Corrosion Testing - Go into salt spray testing and SCC testing to evaluation different spring materials.
Protective Coatings & Inhibitors - Inhibitors – CPCs – phosphates, [chromates, nitrates, molybdates] oxidizers and amines & hydrazines (organic)
Coatings – organic, and zinc, cadmium, and chromium plating
Environment – Natural, Chemical, Storage/Transit
Stress – Residual Stress from Fabrication, Operating Stress (Static, Variable or alternating)
Compatibility – Metals with Metals, Metals with other materials and Quality Control
Movement – Flowing fluids, parts moving in fluids and two phase fluids
Temperature – Oxidation, scales; Heat transfer effects; and Condensation and/or dewpoint
Control – Surface cleaning, Coatings, Cathodic protection, Inhibitors, Inspection and planned maintenace.
Provides some basic corrosion protection. Usually performed for identification purposes. Cautionary note: make sure that the spring are not cleaned in harsh acids before painting --à H2 embrittlement.
Provides some basic corrosion protection, usually oiled afterwards.
Can provide good corrosion protection – until the coating chips off! Electro-statically applied which can lead to arcing.
Can provide good corrosion protection – until the coating chips off! Electro-statically applied which can lead to arcing. Springs must be baked immediately after plating to remove hydrogen. Common practice is 400°F for 4 hours. Refer to ISO-9588-1999. All things being equal, I would recommend using stainless steel over plating.
This can keep processing steps to a minimum – no exposure to hydrogen after the stress relief.
Carbon residue can be formed on the wire surface if the springs are not cleaned before the stress relief or age. The presence of carbon on 302 or 17-7 degrades the corrosion resistance of the alloy.
Low frequencies, especially at low strain amplitudes or when there is substantial elapsed time between changes in stress levels, allow time for interaction between material and environment. On the other hand, high frequencies do not allow for this time of interaction to occur, particularly when high strain amplitude is involved.
The initiation and propagation of corrosion-fatigue cracks is increased by introduction of a corrosive environment. Bulk aqueous solutions and those environments that produce either continuous or intermittent corrosive vapor condensation on the affected surfaces are mainly affected.
This effect varies widely, depending primarily on the characteristics of the material-environment combination. Environment affects crack-growth rate and probability of fatigue-crack initiation, or both. For many materials, the alternating stress range required to cause fatigue failure diminishes progressively with time and with the number of cycles.
This is a zinc plated spring that was used in a salt water marine environment – There are multiple complete fractures which indicates that the was extensive pitting/ extensive corrosion. There is Red rust (iron oxide) corrosion and note the white Zinc oxide and the corrosion pit (shown in the following SEM Photograph) at the fracture origin. The presence of the white zinc oxides indicates that there is still some zinc left in the spring material. The area of the iron oxides indicates that most or all of the zinc has been dissolved due to its use as a sacrificial anode. Zinc plating of springs are used to protect the base metal of the spring, which lead to it useful being extended.
Close up of the spring. The white zinc oxide is all over the spring.
17-7 PH compression springs that controlled relief valves on propane trucks. Springs failed due to hydrogen sulfide (H2S) embrittlement. The H2S was from contaminated propane. When moisture is also present, the sulfur and moisture react to create an acid that is a very corrosive environment for the spring. SCC of the spring results.
Close up of the top spring. Note the whitish film on the spring. This is believed to be sulfur. There presence of moisture and S created caused SCC (Sulphic Acid)
Piece of the spring that came of the previous slide. Intergranular Failure
The SEM Image analysis showed evidence of a brittle fracture which originated at the ID where the highest stresses are normally located.
EDS of the surface deposit confirmed that the Steel was 17-7 and that a significant amount of sulfur was present causing the spring to fail in a brittle, intergranular manner. Sulfur and moisture form sulfuric acid.
SEM image analysis of a 302L spring that was used in a valve assembly. The failure initiated at the spring OD, which is unusual because the stresses are always much lower than those found at the ID. Springs usually break at the ID and therefore when it doesn’t break there, a good reason for such an unusual failure always exists. Pitting corrosion caused this failure and it is plainly visible on the OD.
This is fracture of interest. Note the whole surface shows many pits and a fracture started at one of them. Ratchet marks are visible on fatigue fracture surface indicating the multiple origins where the fracture originated.
Here a view of the spring OD. The highlighted area show extensive corrosion pitting. An examination of this pitted area (corrosion products are shown in the EDS in the next slide.
The EDS confirms the alloy as 302 SS and there is a significant amount of chloride present which is primarily responsible for the corrosion pitting.
