New processincreasecorrresist 11-11-09

New Processes to Increase Corrosion Resistance on Stainless Steel

Siegfried Pießlinger-Schweiger Stainless Steel Congress, Maastricht, November 11th, 2009 page 1 of 35


Siegfried Piesslinger-Schweiger; POLIGRAT GmbH



Content

New Processes to improve the Corrosion Resistance on Stainless Steel

– Basics– Processes– Applications



The Corrosion Resistance of Stainless Steel

is determined by the

1. Alloy and Structure of the Base Metal2. Passive Layer3. Environment

Interacting with Oxygen from environment the Chrome of the Base Metalforms the Passive Layer consisting of Chrom Oxides, which denselycovers and protects the surface of the Base Metal.

Consequence:As long as Stainless Steel does not corrode, only the Passive Layer getsinto contact with the environment. The properties of the Passive Layer determine the behavior of the Stainless Steel surface.



The System providing Corrosion Resistance

The Base Metal

Alloy and Structure provide the basic conditions for the formation of the Passive Layer. They influence the corrosion resistance indirectly.

The Passive Layer

Chemistry, Structure and electrical properties of the Passive Layer formed on the surface of Stainless Steel by Alloy and Crystalline Structure directly influence the corrosion resistance.



Well known Facts Concerning the Passive Layer

The Passive Layer is a non-metallic, oxidic and crystalline layer of about 10 nm thickness.

The Passive Layer contains Chrome Oxide and Iron Oxide in a Cr/Fe ratio of 1:2 – 2:1 depending on the surface condition.

The Passive Layer is a semiconductor, which is conducting electrons and insulating metal ions.



Additional Facts Concerning the Passive Layer

1. The Iron in the Passive Layer is not firmly fixed to oxides. It can be set free and is able to migrate within the Passive Layer.

2. The Iron in the Passive Layer plays an active role in the corrosion mechanism of Stainless Steel.

3. The crystalline structure of the Passive Layer influences the corrosion mechanism.

4. The Passive Layer also contains metallic Chrome and Iron besides the oxides.



Influence of Iron (POLINOX-Protect)

A new method (POLINOX-Protect) allows the extraction of most (> 85%) of the Iron from the existing Passive Layer without removing it.

The extraction of Free Iron from the whole Passive Layer down to the Base Metal produces a Cr/Fe-ratio in the range of 6:1 to 8:1 depending on temperature and duration of application.

The resulting effect shows a significant increase of the pitting corrosion potential as well as of the rest corrosion potential.

The resistance to temperature discoloration (thermic oxidation) increases for about 150°C.

The incubation time for stress corrosion cracking increases at least for thefactor 2.



Chrome-Iron Ratio on S.S. AISI 316 Dependingon Treatment

untreated HNO3 Protect

AISI 316cold rolled

AISI 316electropolished

AISI 316ground grain 240

Cr/

Fe

ra

tio



Chrome-Iron Ratio through the Passive LayerC

r/F

e r

ati

o

untreated with POLINOX-Protect

Passive Layer Base Metal

Layer thickness in nm



Pitting Corrosion Potential on AISI 304 Treated with POLINOX-Protect at Different Temperatures and Different Periods of Time



Pitting Corrosion Potential of S.S. AISI 304according to Surface Finish

AISI 304cold rolled



pitting corrosion potential AISI 304

POLINOX-Protectuntreated



Open Circuit Potential of S.S. AISI 316 with and without Passivation with POLINOX-Protect

AISI 316cold rolled



open circuit potential AISI 316

POLINOX-Protectuntreated



Corroded S.S. AISI 316 before and after Cleaning with POLINOX-Protect Applied on Site by Wiping and Staying on for 3 Hours at 20 °C



Descaling of Welding Seams by POLINOX-Protect

Scale and discolorations in the area of welding seams and heat affected zones consist of metal oxides in the ratio close to the alloy.

POLINOX-Protect selectively cracks the Iron Oxides (except hematite) and removes the Iron preserving the oxide layer.

After removal of Iron the Oxide Layer preferently is formed by Chrome Oxide and shows a comparable high corrosion resistance as the not heat-affected and passivated areas.

Consequently, treatment with POLINOX-Protect may partially replace pickling.



S.S. AISI 304 with Welding Seam Partially Treated with POLINOX-Protect at 50 °C for 3 Hours



S.S. AISI 316 Ti with Welding Seam before and after Treatment with POLINOX-Protect at 70 °C for 3 Hours



Comparison of Pitting Corrosion Potential at Base Metal, Heat Affect Zone and Welding Seam on AISI 316Ti, 20.000 ppm Chloride



Influence of Crystalline Structure (POLINOX-Protect TC)

Based on the idea, that the migration of Ions would mostly happen along crystal borders, POLIGRAT applied heat treatment to produce changes within the Passive Layer.

The treatment was performed on different Alloys using different temperatures below the level of thermic discoloration and for different times of application.

The resulting effects show a significant increase in corrosion potentials specifically after Iron extraction by POLINOX-Protect. Depending on structure (ferritic or austenitic) an individual optimum exists in view of temperature and time of treatment.



Pitting Corrosion Potential of S.S. AISI 304 (Austenitic) Treated with POLINOX-Protect TC at Different Temperatures



Pitting Corrosion Potential of S.S. AISI 304 (Austenitic) Treated with POLINOX-Protect TC at 200 °C Depending on Time for Heat Treatment



Pitting Corrosion Potential of S.S. AISI 430 (Ferritic) Treated with POLINOX-Protect TC at Different Temperatures



Pitting Corrosion Potential of S.S. AISI 430 (Ferritic) Treated with POLINOX-Protect TC at 140 °C Depending on Time for Heat Treatment



Pitting Corrosion Potential on S.S. – Surface Polished with Grain 240

untreated HNO3POLINOX-Protect

AISI 430 AISI 304 AISI 316 Ti AISI Alloy 59

POLINOX-Protect TC



Stress Corrosion Cracking on AISI 321 (hours)

0

20

40

60

80

100

untreated passivateduntreated Protect TC



POLINOX-Protect TC

POLINOX-Protect TC combines the application of POLINOX-Protect with a following controlled heat treatment.

POLINOX-Protect TC significantly increases the corrosion resistance.

The optimum temperature is in the range of 140 °C (for ferritic S.S.) and 220 °C (for austenitic S.S).

The optimum time for heat treatment is about 5 to 15 minutes for ferritic Stainless Steel and up to 60 minutes for austenitic Stainless Steel.

The heat treatment can also be realized at a later date.

The effect is not reversible.



POLINOX-Protect – Properties and Application

POLINOX-Protect is applied as dipping process, spray-on paste or brush paste for a period of 4 – 5 hours at temperatures from 20 °C to 70 °C. Following, the surfaces are cleaned with water.

POLINOX-Protect is based on a special combination of organic complexing and chelating agents. It is fully organic, biodegradable, not hazardous, not fuming or smelling and is not enriched with heavy metals during application.

POLINOX-Protect can be applied without restriction to all grades of Stainless Steel with a chromium content higher than 15 % on new parts and for maintenance. Alloys with a lower chromium content can be treated after definition of parameters by prior sample test.

POLINOX-Protect can be used for cleaning and corrosion protection by the same treatment.



Effects of POLINOX-Protect on Stainless Steel

Improving the resistance to pitting corrosion, ferrite induced corrosion and stress corrosion cracking (corrosion protection).

Increasing the resistance to thermal discoloration.

Removal of ferritic contamination, rust and corrosion products and restoration of corrosion resistance on a higher level (refurbishment).

Restoring the corrosion resistance of welding seems and heat affected zones.

Reduction of rouging.

Preserving the surface finish unchanged.

Locally damaged surfaces repassivate at the same level of corrosion resistance.



Practical Experience

The new Passivation Process is in field application for about five yearscomprising a wide range of parts:

Automotive body parts Architecture Food processing machines Street furniture Fasteners and springs Pharmaceutical and chemical processing plants Swimming pools

The new process is applied on new parts as well as on used parts forrepair and reconditioning.



Case Histories

1. Hand rails made of S.S. tube, material AISI 304, polished grain 240, placed along the Atlantic seashore stained and corroded heavily and had to be cleaned every 4 - 6 weeks. After cleaning with POLINOX-Protect for more than 1 year no corrosion has reappeared.

2. Body parts of cars including front and bonnet made of S.S. 304 with brushed surface are passivated by POLINOX-Protect. During production for a period of three years no corrosion appeared despite exposure to deicing salts.



Rolls Royce Hood made from Stainless Steel 304 Passivated by POLINOX-Protect



Case Histories

3. Fishing hooks made of Stainless Steel 430 showed insufficient corrosion resistance to sea water even after electropolishing (salt spray less than 25 hours). POLINOX-Protect TC (dipping for 3 hours at 60 °C and heating to 140 °C for 5 minutes) improved the corrosion resistance to > 500 hours.



POLINOX-Protect TC

Fishing Hooks



Conclusions

There is a new approach to corrosion resistance of S.S. by considering S.S. as a „composite“ consisting of a metallic base and a non-metallic protective layer, the Passiv Layer.

Research on the influence of chemistry and structure of the Passive Layer to the corrosion resistance of S.S. has revealed a surprisingly complex and variable system.

The corrosion resistance of S.S. can be significantly improved by upgrading the Passive Layer independently from the base metal.



Conclusions

A special Passivation Process has been developed to improve chemistry and structure of the Passive Layer. It provides substantial improvements in a wide range of corrosion effects.

The Passivation Process is save, easy and proven in application. It is not hazardous, biodegradable, and easy to apply.

The R&D work is not finished yet. There is still a lot to do!!



Thank You for Listening

New processincreasecorrresist 11-11-09

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