Table of Contents
22.214.171.124 Measures to Prevent Damage due to Stress Corrosion Cracking
SCC can not develop if one succeeds in removing one of the following three prerequisites from the corrosion system:
- sensitive material
- critically high tensile stresses
- material-specific corrosive media
All remedies must be oriented to this requirements.
- Selection of materials that are SCC resistant, i.e. determining semi-finished product production, manufacturing processes, and heat treatments that prevent the development of structures that promote SCC. Heavily soiled materials such as free-machined materials (with impurities as chip breakers).
This is especially true for corrosion types that involve hydrogen creation.
- The highest possible material strength should not be used, neither through loading or production (e.g. shaping, heat treatment). For example, in Cr steels, hardness above 30 HRC is an indicator for possible SCC-sensitivity.
- Depositing hand sweat before the material is subject to temperatures above 500°C must absolutely be avoided. This may occur during manufacture before heat treatment, and should be preventable by wearing suitable gloves.
- Care must be taken during corrective cold-forming of sensitive materials (such as when repairing damages through straightening and non-chipping shaping). Dangerous local residual stresses and/or structural changes may occur.
- The material should not be loaded across the direction of fiber (rolling, forming direction when forging). This is especially true for SCC-sensitive Al alloys and Cr steels.
- Reducing tension residual stresses from manufacture as much as possible through suitable heat treatment (low-tension baking).
- Inciting compressive internal stresses at the surface, such as through shot peening or rolling. Using chipping procedures with set parameters (e.g. chipping rate, chip depth, tool geometry, cool cutting media), for which compressive stresses have been verified after chipping. No changes to procedures or parameters without verification of comparable compressive stresses in the surface being machined.
Experience has shown that grinding will create tensile stresses in surfaces. For this reason, grinding should, if possible, be avoided in systems that are prone to SCC.
- Certifying and monitoring auxiliary materials during manufacture (e.g. cutting fluid, cleaning and etching baths, etc.). Special caution with auxiliary materials with halogen compounds such as chlorine. If in doubt, these products should be certified in suitable SCC tests.
Experience has shown that the following combinations of processing bath and material require special caution:
- bronzing/burnishing baths with hardened and case-hardened steel parts
- hot perchloroethylene degreasing baths with high-strength Ti alloys
- Rinsing baths contaminated with chloride combined with parts made from high-strength Al alloys and (martensite) Cr steels.
In every case, sensitive parts (such as parts made from case-hardened steels such as gears and shafts) should be sufficiently shot peened on all surfaces that are to be treated with a bath.
- Realistic and suitable testing of materials (e.g. goods receipt) and parts (patterns, part testing) for SCC-sensitivity to manufacturing and operating influences.
- Weld seams should be located in non-critical areas. These include areas that are not under high tensile stresses, or those in which no electrolyte fluid can collect.
- Suitable realistic corrosion systems (material, loads/sample shape, media) must be selected in order to verify SCC risks. Defining the characteristic (e.g. crack growth rate, time to crack initiation, time to fracture) is important for selecting a test for SCC-sensitivity. For example, SCC testing of Ti alloys frequently requires nicked or cracked samples, with the crack growth speed and crack image as criteria.
- Use of electrochemical testing methods (e.g. measure potential) in order to determine SCC-sensitivity.
- Hollow spaces should be avoided as early as the design phase. In rotor parts such as disks and spacers, these are conical lugs in which corrosive media can collect during standstill or under the influence of centrifugal force.
In blind holes, the hole and threads must be sufficiently sealed against operating fluids. It must be ensured that no corrosive remnants (e.g. from galvanic baths or etching and cleaning baths) remain in the threaded blind holes, which might later react with other fluids during operation (such as condensation water) and form dangerous electrolytes.
- Selection of suitable coatings, such as organic lacquers and inorganic coatings (Al powder-filled lacquer with a ceramic binder, enamel, metallic coatings). Cathodic protective effects are desirable, since they provide corrosion protection of the base material even if the coating is damaged all the way through.
- Avoiding sharp corners and edges, as well as crack-like flaws, around which stress peaks can be expected.
- The surfaces should be as smooth an clean as possible. Contaminants that can change the corrosion behavior (e.g. cell action), such as may be deposited during manufacturing or repair procedures (e.g. smeared tool material, remnants of peening material), must be avoided or safely removed.
- With “catalog parts”, special care is necessary to ensure that:
- the material is in the most SCC-resistant state possible with regard to the expected media
- no additional sensitivity is created due to a combination with other parts (e.g. cell action)
- quality control must meet aviation standards
- the unique operating conditions on the engine are taken into account during design and selection.
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