#2 Defining Rust Back, Flash Rust, and Rust Bloom
Rust Back looks like Rust Bloom during dry abrasive blast cleaning Fig 3
There is marine salt at top of the Rail car that contaminated the ricochet abrasive.
Rust Back appears as Rust Bloom within hours.
Figure 3 shows a rail car that was prepared by dry abrasive blast cleaning in a room where humidity and temperature were controlled. Normally, this facility could hold the blaston five cars before painting them. The rust bloom along the top half of the car occurred within an hour of blasting. This is rust back, not flash rust. First, flash rust occurs with wet blasting; dry blasting was used in this case. Second, the rust only occurred on the top half, even though the bottom half was subjected to the same environmental conditions. These factors indicate that the rust back was probably accelerated by salt contamination. The rust back is evenly dispersed, which indicates that salt was probably spread by contaminated abrasive. Another indication that the rust back was most likely influenced by salt is that where the rust was heavy, it could not be easily removed. It would be prudent in this case to perform a test for soluble salts, followed by possible salt remediation before painting.
Rust Back from dry abrasive blast cleaning (Fig 4)
This is the other side of the Rail Car.
The appearance is localized, dark black, splotchy.
Is it Rust Bloom by definition that Rust Bloom is “new rust?”
I call it BAD RUST!
Rust Back within a few hours from salt contamination.
The other side of the same rail car exhibiting rust back after dry abrasive blast cleaning is shown in Fig. 4. Here, rust back also occurred within an hour after blasting, but the rusting is localized, so it is not rust bloom. As with the rail car in Fig. 3, this localized rust back was most likely accelerated by salt contamination remaining on the surface in the bottom of pits.
Figures 3 and 4 show rust back caused by salt contamination after dry abrasive blast cleaning. The uniform rust back (rust bloom) in Fig. 3 was caused by contaminated abrasive, and the localized rust back in Fig. 4 was caused by surface contamination with localized salt corrosion cells. To apply paint over either surface involves high risk.
Before any meaningful dialog about rust back, flash rust, and rust bloom can occur, there must be agreement on the precise meaning of these terms. Understanding the root causes of the forms of rust described above will lead to informed decisions about which rusted surfaces can be painted and which ones should be re-cleaned, thereby avoiding unnecessary cleaning operations as well as costly coating failures.