Car MOT Manual 2012 onwards
The effect of corrosion on the safety of a vehicle depends on the extent of the corrosion and the function of the section or component on which the corrosion has occurred.
A small amount of corrosion in an important part of a vehicle structure can make a vehicle unsafe where it compromises the load bearing capacity of the structure. On the other hand, excessive corrosion on non structural sections may have little or no effect on the vehicle’s safety.
Corrosion of a particular part, such as a door sill, may be very important on one type of vehicle construction, but of less importance on another. This can be seen in figures 1 to 4 at the end of this Appendix, where the shaded portions indicate the important load bearing parts of various typical vehicle constructions.
2. Prescribed areas
Certain areas of the vehicle structure are particularly important for the safety of a vehicle. Particular attention must be paid to these areas during an inspection. These areas are:
• the load bearing parts of the vehicle to which the testable items defined in Sections 2, 3 and 5 of the Inspection Manual are mounted, and
• any load bearing or supporting structure or supporting panelling within 30cm of the mounting location.
To give an example; during the examination of a seat belt mounting on an inner sill, consideration must be given to the outer sill (or the sill reinforcement if the outer sill is a plastic cover), door pillar, floor panel or any other structural part within 30cm of the component’s mounting point. It is accepted that it is not possible to assess some of these areas due to the fitment of body trim etc.
3. Corrosion assessment
Having identified the important load bearing members and ‘prescribed areas’ on a vehicle, the tester should determine whether they are excessively corroded, firstly by visual inspection and then by finger and thumb pressure.
If necessary, the Corrosion Assessment Tool should be used to assess the extent of any corrosion by careful scraping or light tapping of the affected areas.
It is important that use of the Corrosion Assessment Tool is restricted to ascertaining that the failure criteria are met and not used for heavy scraping or poking of the affected areas.
Excessively corroded metal, or metal treated with filler, emits a duller sound than unaffected metal. It is not permissible to apply heavy impact blows or to use a sharp instrument to probe at the structure.
4. Failure criteria within ‘prescribed areas’
Corrosion within a ‘prescribed area’ should be rejected if:
• the corrosion has caused a hole in the metal, or
• it is weakened to the extent that by finger and thumb pressure it does not feel sound, or
• finger and thumb pressure or use of the Corrosion Assessment Tool causes a hole.
Additionally, any fracture or inadequate repair (see items 11 to 14) within a ‘prescribed area’ is a Reason for Rejection. However, deliberate modification or severe distortion within a ‘prescribed area’ should only be rejected if the strength of a component mounting, load bearing member, supporting structure or supporting panelling is significantly reduced
5. Failure criteria not within prescribed areas
Structural fractures, damage or corrosion not within the ‘prescribed areas’ are dealt with in Section 6 of the Manual. The diagrams at figures 1 to 4 show the main load bearing members of various types of vehicle construction which need to be considered.
6. Highly stressed components
The severity of general or local corrosion in highly stressed components, such as steering and suspension arms, rods, levers etc. can be assessed by lightly tapping or scraping with the Corrosion Assessment Tool.
In places inaccessible to the Corrosion Assessment Tool, an alternative blunt instrument may be used. A highly stressed component should be rejected if corrosion has resulted in serious reduction in the overall thickness of the material, or has caused a hole or split.
Welded repairs to highly stressed components such as steering/suspension arms, rods, levers etc are not normally acceptable, other than where the component is made up of sections that are welded together. In these cases the repair should appear to be as strong as the original design.
7. Thin gauge steel pressings
It is common to use thin gauge pressings for certain steering and suspension components, mountings, sub-frames and cross members.
These are designed to have minimal material usage with maximum strength, resulting in hollow or open sections or ‘up facing’ areas in which road dirt impregnated with salt or other chemicals collects causing serious but often very localised corrosion.
These types of components require extra vigilance as there may be little evidence of corrosion on initial inspection, but may reveal severe material failure when looked at more closely. Cont’d Appendix C – Structural Integrity and Corrosion
8. Vehicles with separate bodies
Some vehicle types have bodies and various mechanical components attached to a separate under frame. The frame is the main load bearing structure with a passenger cell and possibly a separate load bed secured on top of the frame, which may also be load bearing or supportive.
Excessive corrosion within these body panels should only lead to rejection if the failure criteria are met for:
• prescribed areas, or
• is likely to adversely affect the correct functioning of the braking or steering, or
• body security.
9. General guidance
The presenter should be advised of any corrosion or damage which has not yet reached the stage where it meets the criteria for rejection. However, it may be necessary to refuse to test the vehicle if excessive damage or corrosion could result in injury or further damage to the vehicle and/or testing facility.
10. Acceptable methods of repair
It is essential that repairs to corroded or damaged areas are properly carried out and appear to be as strong as the original structure.
This requires that suitable materials of appropriate gauge or thickness are used and any plating or welding extends to a sound part of a load bearing member.
Where a load bearing panel was originally spot welded, it is acceptable for a replacement panel or patch repair to be spot welded along that flange, provided the original panel or section has been removed.
In all other circumstances, patch repairs must be continuously seam welded.
Stitch or plug welding is an acceptable alternative to spot welding
Some vehicle manufacturers have recommended repair methods that use MIG brazing, a combination of adhesive bonding and riveting, or amalgamations of these with other joining methods. Such repairs must therefore be accepted unless they are clearly inadequate.
11. Unacceptable methods of repair
Gas brazing, soldering, adhesive bonding, fibre reinforcement and body filler are bonding processes and are not regarded as strong enough for repairs to load bearing members although they are normally adequate for other repair work.
It is accepted that it is sometimes difficult to identify the repair method after the repair has been covered in paint or underseal. If the method of repair is in doubt the tester should pass and advise.
12. Testable items mounted to plastic structures
Testable items, such as steering racks, sub frames and seat belts, are sometimes mounted directly to plastic structures that do not have metal reinforcement.
Non-metallic structures must be assessed on their merits. Evidence of weakness such as cracks, separation or delamination within a ‘prescribed area’ or a component flexing on its mounting to the extent that it is clear that it is likely to work loose or break away are reasons for rejection.
13. Repairs to non-metallic load-bearing structures
Repairs to non-metallic structures are not acceptable within prescribed areas. Any repairs outside of these areas should be as strong as the original structure.
14. Panel removal or replacement with different materials
On a vehicle of integral construction the strength and stiffness of the whole structure may be seriously affected by any panel being removed or replaced by a panel of different material.
If it is considered that a modification has significantly reduced the original strength and stiffness of the structure, the tester should fail the vehicle under the appropriate section. If there is doubt about the effect of the modification the tester should pass and advise.
It is unacceptable for plastics to replace or reinforce corroded or weak metal in prescribed areas and load bearing sections